HYDROARYLATION OF OLEFINS TECHNICAL FIELD [0001] The present invention relates to a process for hydroarylation of an olefin in the presence of a lanthanide complex. The invention further relates to a lanthanide complex and process for the preparation thereof. BACKGROUND ART [0002] The addition of alkyl groups to aromatic systems is a vital transformation in both academic and industrial contexts, providing access to a wide variety of synthetic intermediates, fine chemicals and feedstocks. [0003] As benzene does not typically undergo nucleophilic substitution (S _N 1 or S _N 2), alkylation of benzene is conventionally achieved by the electrophilic aromatic (Friedel-Crafts) substitution of a benzene C-H bond using a strong Lewis acid catalyst. Electrophilic aromatic substitution reactions rely on using reactive alkylating agents such as alkyl halides and generate stoichiometric by-products. Consequently, while efficient, current processes involving Friedel- Crafts alkylation are inevitably hampered by the cost of expensive halogenated starting materials and the generation of stoichiometric waste. [0004] An alternative to Friedel-Crafts alkylation is hydroarylation. Such reactions involve addition of an arene C-H bond across olefins and offer significant advantages over classical Friedel-Crafts alkylation. For example, the reaction is by-product free and the olefin starting materials are generally cheaper and more readily available than the corresponding alkyl halide. However, current hydroarylation processes still suffer from disadvantages such as: high cost of catalyst and poor selectivity to linear chains. [0005] While several potent nucleophilic main group species have recently been shown to activate and functionalise aryl C-H bonds, only transition metal complexes have provided an alternative catalytic pathway to new alkylated arene products. Hydroarylation of olefins catalysed by Ru, Rh, Ir, Pt, Ni, Co and Fe has been extensively studied and, in each case, transition metal alkyl complexes have been identified as catalytically important intermediates that operate via a non-Friedel-Crafts mechanism. Existing rhodium-based catalysts are the current gold standard for the hydroarylation of olefins. These transformations, however, are heavily reliant on directing groups to achieve appreciable levels of site selectivity and suffer from poor specificity toward the formation of branched vs. linear (i.e. Markovnikov vs. anti- Markovnikov) arene products. The selective hydroarylation of arenes, such as benzene, to linear (anti-Markovnikov) alkyl arene products, therefore, remains a significant challenge. [0006] Accordingly, it is an object of the present invention to go some way to avoiding the above disadvantages; and/or to at least provide the public with a useful choice. [0007] Other objects of the invention may become apparent from the following description which is given by way of example only. [0008] Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is solely for the purpose of providing a context for the present invention. It is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed before the priority date. SUMMARY OF THE INVENTION [0009] In a first aspect, the present invention provides a process for preparing a compound of formula I, or an isotopologue thereof, the process comprising reacting a compound of formula II, or an isotopologue thereof, II with a compound of formula III, or an isotopologue thereof, in the presence of a lanthanide complex, wherein R ₁ , R ₂ , R ₃ and R ₄ are each independently selected from the group consisting of H, alkyl, alkenyl, alkynyl, cycloalkyl, cycloalkenyl, heteroalkyl, heteroalkenyl, cycloheteroalkyl and cycloheteroalkenyl, provided that at least one of R ₃ and R ₄ is H; R _x is H; and R ₅ , R ₆ , R ₇ , R ₈ and R ₉ are each independently selected from the group consisting of H, alkyl, alkenyl, alkynyl, cycloalkyl, cycloalkenyl, heteroalkyl, heteroalkenyl, cycloheteroalkyl and cycloheteroalkenyl; or R ₁ and R ₅ taken together form –X-(CH ₂ ) _n – wherein X is C, O, NR _a or S, R _a is H or alkyl, and n is 0, 1, 2 or 3; R ₂ , R ₃ and R ₄ are each independently selected from the group consisting of H, alkyl, alkenyl, alkynyl, cycloalkyl, cycloalkenyl, heteroalkyl, heteroalkenyl, cycloheteroalkyl and cycloheteroalkenyl, provided that at least one of R ₃ and R ₄ is H; and R ₆ , R ₇ , R ₈ and R ₉ are each independently selected from the group consisting of H, alkyl, alkenyl, alkynyl, cycloalkyl, cycloalkenyl, heteroalkyl, heteroalkenyl, cycloheteroalkyl and cycloheteroalkenyl; wherein each of said alkyl, alkenyl, alkynyl, cycloalkyl, cycloalkenyl, heteroalkyl, heteroalkenyl, cycloheteroalkyl and cycloheteroalkenyl is optionally substituted with one or more substituents independently selected from the group consisting of hydroxyl, halogen, carbonyl, carboxyl, alkoxy, amino, amido, thiol and nitro. [0010] Preferably, the lanthanide complex is a compound of formula IV wherein M is selected from the group consisting of ytterbium, europium, thulium, dysprosium and samarium; R ₁₀ and R ₁₄ are the same and are selected from the group consisting of aryl and heteroaryl; R ₁₁ and R ₁₃ are the same and are selected from the group consisting of H, alkyl, alkenyl, alkynyl, cycloalkyl, cycloalkenyl, heteroalkyl, heteroalkenyl, cycloheteroalkyl and cycloheteroalkenyl; R ₁₂ is H; and wherein each of said aryl, heteroaryl, alkyl, alkenyl, alkynyl, cycloalkyl, cycloalkenyl, heteroalkyl, heteroalkenyl, cycloheteroalkyl and cycloheteroalkenyl is optionally substituted with one or more substituents independently selected from the group consisting of hydroxyl, halogen, alkyl, carbonyl, carboxyl, alkoxy, amino, amido, thiol and nitro. [0011] Preferably, R ₁₀ and R ₁₄ are substituted aryl, more preferably, R ₁₀ and R ₁₄ are the same and are selected from the group consisting of 2,6-diisopropylphenyl and 2,6-diisopentylphenyl. [0012] In some embodiments, R ₁₁ and R ₁₃ are selected from the group consisting of C _1-6 alkyl, CF ₃ and aryl. In some embodiments, R ₁₁ and R ₁₃ are C _1-6 alkyl. Preferably, R ₁₁ and R ₁₃ are methyl. [0013] Preferably, M is ytterbium. [0014] In some embodiments, the lanthanide complex is a compound of formula IVa [0015] In some embodiments, R ₁ is unsubstituted C _1-6 alkyl. [0016] In some embodiments, R ₂ , R ₃ and R ₄ are H. [0017] In some embodiments, R ₅ , R ₆ , R ₇ , R ₈ and R ₉ are each independently selected from the group consisting of H, C _1-6 alkyl and C _1-6 alkenyl. [0018] In some embodiments, R ₅ , R ₆ , R ₇ , R ₈ and R ₉ are H. [0019] In some embodiments, the isotopologue of the compound of formula II is a deuterated isotopologue. Accordingly, in some embodiments, one or more of R ₂ , R ₃ and R ₄ are D. In one embodiment R ₂ , R ₃ and R ₄ are D. [0020] In some embodiments, the isotopologue of the compound of formula III is a deuterated isotopologue. Accordingly, in some embodiments, one or more of R ₅ , R ₆ , R ₇ , R ₈ and R ₉ are D, for example, wherein R ₅ , R ₆ , R ₇ , R ₈ and R ₉ are D. [0021] In some embodiments, the compound of formula I is a compound of formula Ia, or an isotopologue thereof, and the compound of formula III is a compound of formula IIIa, or an isotopologue thereof, IIIa wherein n is 0 or 1. [0022] In some embodiments, the reaction is performed in a non-polar solvent. In some embodiments, the non-polar solvent is selected from the group consisting of pentane, hexane and cyclohexane, benzene, an alkylated benzene, a haloaryl solvent and nitrobenzene. [0023] In a further aspect, the present invention provides a compound of formula I, or an isotopologue thereof, when prepared by a process of the first aspect. [0024] In a still further aspect, the present invention provides a lanthanide complex of formula IV IV wherein M is selected from the group consisting of ytterbium, europium, thulium, dysprosium and samarium; R ₁₀ and R ₁₄ are the same and are selected from the group consisting of aryl and heteroaryl; R ₁₁ and R ₁₃ are the same and are selected from the group consisting of H, alkyl, alkenyl, alkynyl, cycloalkyl, cycloalkenyl, heteroalkyl, heteroalkenyl, cycloheteroalkyl and cycloheteroalkenyl; R ₁₂ is H; and wherein each of said aryl, heteroaryl, alkyl, alkenyl, alkynyl, cycloalkyl, cycloalkenyl, heteroalkyl, heteroalkenyl, cycloheteroalkyl and cycloheteroalkenyl is optionally substituted with one or more substituents independently selected from the group consisting of hydroxyl, halogen, alkyl, carbonyl, carboxyl, alkoxy, amino, amido, thiol and nitro. [0025] In another aspect, the present invention provides a process for preparing a lanthanide complex of formula IV, the process comprising reacting a compound of formula Va with PhSiH ₃ , wherein wherein M is selected from the group consisting of ytterbium, europium, thulium, dysprosium and samarium; R ₁₅ is silyl substituted with one, two or three substituents independently selected from alkyl and aryl; R ₁₀ and R ₁₄ are the same and are selected from the group consisting of aryl and heteroaryl; R ₁₁ and R ₁₃ are the same and are selected from the group consisting of H, alkyl, alkenyl, alkynyl, cycloalkyl, cycloalkenyl, heteroalkyl, heteroalkenyl, cycloheteroalkyl and cycloheteroalkenyl; R ₁₂ is H; and wherein each of said aryl, heteroaryl, alkyl, alkenyl, alkynyl, cycloalkyl, cycloalkenyl, heteroalkyl, heteroalkenyl, cycloheteroalkyl and cycloheteroalkenyl is optionally substituted with one or more substituents independently selected from the group consisting of hydroxyl, halogen, alkyl, carbonyl, carboxyl, alkoxy, amino, amido, thiol and nitro. [0026] In another aspect, the present invention provides a process for preparing a lanthanide complex of formula IV, the process comprising reacting a compound of formula Vb with 1,4-cyclohexadiene, wherein M is selected from the group consisting of ytterbium, europium, thulium, dysprosium and samarium; R ₁₆ is silyl substituted with one, two or three substituents independently selected from alkyl and aryl; R ₁₀ and R ₁₄ are the same and are selected from the group consisting of aryl and heteroaryl; R ₁₁ and R ₁₃ are the same and are selected from the group consisting of H, alkyl, alkenyl, alkynyl, cycloalkyl, cycloalkenyl, heteroalkyl, heteroalkenyl, cycloheteroalkyl and cycloheteroalkenyl; R ₁₂ is H; and wherein each of said aryl, heteroaryl, alkyl, alkenyl, alkynyl, cycloalkyl, cycloalkenyl, heteroalkyl, heteroalkenyl, cycloheteroalkyl and cycloheteroalkenyl is optionally substituted with one or more substituents independently selected from the group consisting of hydroxyl, halogen, alkyl, carbonyl, carboxyl, alkoxy, amino, amido, thiol and nitro. [0027] Preferably, R ₁₀ and R ₁₄ are substituted aryl, more preferably, R ₁₀ and R ₁₄ are the same and are selected from the group consisting of 2,6-diisopropylphenyl, 2,6-diisopentylphenyl. [0028] In some embodiments, R ₁₁ and R ₁₃ are selected from the group consisting of C _1-6 alkyl, CF ₃ and aryl. Preferably, R11 and R13 are methyl. [0029] In some embodiments, R ₁₅ is silyl substituted with one, two or three substituents independently selected from C _1-12 alkyl and C _6-12 aryl, preferably C _1-6 alkyl and phenyl. Preferably R ₁₅ is tri(C _1-12 alkyl)silyl, more preferably tri(C _1-6 alkyl)silyl. In some embodiments, R ₁₅ is trimethylsilyl, triethylsilyl, triisopropylsilyl, tert-butyldimethylsilyl or tert-butyldiphenylsilyl, preferably trimethylsilyl. [0030] In some embodiments, R ₁₆ is silyl substituted with one, two or three substituents independently selected from C _1-12 alkyl and C _6-12 aryl, preferably C _1-6 alkyl and phenyl. Preferably R ₁₆ is tri(C _1-12 alkyl)silyl, more preferably tri(C _1-6 alkyl)silyl. In some embodiments, R ₁₆ is trimethylsilyl, triethylsilyl, triisopropylsilyl, tert-butyldimethylsilyl or tert-butyldiphenylsilyl, preferably trimethylsilyl. Preferably, R16 is trimethylsilyl. [0031] In some embodiments, the compound of formula Vb is prepared by reacting a compound of formula VIIIb

with a KCH(R ₁₆ ) ₂ , wherein [0032] R ₁₇ is a halogen.In some embodiments, R ₁₇ is iodine. [0033] In still another aspect, the present invention provides a compound of formula VII wherein M is selected from the group consisting of ytterbium, europium, thulium, dysprosium and samarium; R ₁ , R ₂ and R ₃ are each independently selected from the group consisting of H, alkyl, alkenyl, alkynyl, cycloalkyl, cycloalkenyl, heteroalkyl, heteroalkenyl, cycloheteroalkyl and cycloheteroalkenyl; each R ₁₀ and R ₁₄ are the same and are selected from the group consisting of aryl and heteroaryl; each R ₁₁ and R ₁₃ are the same and are selected from the group consisting of H, alkyl, alkenyl, alkynyl, cycloalkyl, cycloalkenyl, heteroalkyl, heteroalkenyl, cycloheteroalkyl and cycloheteroalkenyl; R ₁₂ is H; and wherein each of said aryl, heteroaryl, alkyl, alkenyl, alkynyl, cycloalkyl, cycloalkenyl, heteroalkyl, heteroalkenyl, cycloheteroalkyl and cycloheteroalkenyl is optionally substituted with one or more substituents independently selected from the group consisting of hydroxyl, halogen, alkyl, carbonyl, carboxyl, alkoxy, amino, amido, thiol and nitro. [0034] In some embodiments, any one or more of the compounds of formula I, II, III and/or IV is an isotopologue, e.g. a deuterated isotopologue. Accordingly, in some embodiments, any one or more of the compounds of formula I, II, III and/or IV is enriched with an unnatural proportion of one or more isotopes. For example, in some embodiments, one or more of R ₁ , R ₂ , R ₃ R ₄ , R ₅ , R ₆ , R ₇ , R ₈ , R ₉ , R ₁₀ , R ₁₁ , R ₁₂ , R ₁₃ , R ₁₄ , R ₁₅ and R _x is or comprises D. In some embodiments, one or more of R ₁ , R ₂ , R ₃ R ₄ , R ₅ , R ₆ , R ₇ , R ₈ , R ₉ and R _x is D. In some embodiments, R ₅ , R ₆ , R ₇ , R ₈ , R ₉ and R _x are D. In some embodiments, R ₂ , R ₃ and R ₄ are D. [0035] This invention may also be said broadly to consist in the parts, elements and features referred to or indicated in the specification of the application, individually or collectively, and any or all combinations of any two or more said parts, elements or features, and where specific integers are mentioned herein which have known equivalents in the art to which this invention relates, such known equivalents are deemed to be incorporated herein as if individually set forth. [0036] In addition, where features or aspects of the invention are described in terms of Markush groups, those persons skilled in the art will appreciate that the invention is also thereby described in terms of any individual member or subgroup of members of the Markush group. [0037] The term “alkyl” as used herein refers to a radical of saturated aliphatic hydrocarbon groups. For example, the alkyl may be a C _1-12 alkyl, i.e. an alkyl having 1 to 12 carbon atoms, a C _1-10 alkyl, a C _1-6 alkyl, a C _1-5 alkyl, a C _1-4 alkyl, a C _1-3 alkyl, or a C _1-2 alkyl. In some embodiments, the alkyl is a straight chain aliphatic hydrocarbon radical, such as methyl, ethyl, propyl, or butyl. In some embodiments, the alkyl group is a branched chain saturated aliphatic hydrocarbon radical, such as isopropyl or isobutyl. [0038] The term “alkenyl” as used herein refers to a radical of unsaturated aliphatic hydrocarbon groups having one or more carbon–carbon double bonds. For example, the alkenyl may be a C _2-6 alkenyl, i.e. an alkenyl having 2 to 6 carbon atoms. In some embodiments, the alkenyl is a straight chain aliphatic hydrocarbon radical, such as ethenyl, propenyl, butenyl or pentenyl. In some embodiments, the C _2-6 alkenyl is a branched chain unsaturated aliphatic hydrocarbon radical, such as isopropenyl or isobutenyl. [0039] The term “alkynyl” as used herein refers to the radical of unsaturated aliphatic hydrocarbon groups having one or more carbon–carbon triple bonds. For example, the alkynyl may be a C _2-6 alkynyl, i.e. an alkynyl having 2 to 6 carbon atoms. In some embodiments, the alkynyl is a straight chain aliphatic hydrocarbon radical, such as ethynyl, propynyl, butynyl or pentynyl. In some embodiments, the C _2-6 alkynyl is a branched chain unsaturated aliphatic hydrocarbon radical, such as propynyl, 1-butynyl or 2-butynyl. [0040] The term “cycloalkyl” as used herein refers to the radicals of cyclic saturated aliphatic hydrocarbon groups. For example, the cycloalkyl may be a C _3-6 cycloalkyl, i.e. a cycloalkyl having 3 to 6 carbon atoms. In some embodiments, the cycloalkyl is cyclopropyl, cyclobutyl, cyclopentyl or cyclohexyl. [0041] The term “cycloalkenyl” as used herein refers to the radicals of cyclic unsaturated aliphatic hydrocarbon groups having one or more carbon–carbon double bonds. For example, the cycloalkyl may be a C _3-6 cycloalkenyl, i.e. a cycloalkenyl having 3 to 6 carbon atoms. In some embodiments, the cycloalkenyl group is cyclopropenyl, cyclobutenyl or cyclopentenyl. [0042] The term “aryl” as used herein refers to an aromatic ring or fused ring system wherein each of the ring atoms is a carbon atom. Aryl rings can be formed by 5, 6, 7, 8, 9 or more carbon atoms. Examples of aryl include, but are not limited to, phenyl and napththyl. [0043] The term “heteroalkyl” as used herein refers to alkyl radicals in which one or more of the skeletal chain atoms is a heteroatom, e.g., oxygen, nitrogen, sulfur, silicon, phosphorus or combinations of one or more thereof. [0044] The term “heteroalkenyl” as used herein refers to alkenyl radicals in which one or more of the skeletal chain atoms is a heteroatom, e.g., oxygen, nitrogen, sulfur, silicon, phosphorus or combinations of one or more thereof. [0045] The term “heteroalkynyl” as used herein refers to alkynyl radicals in which one or more of the skeletal chain atoms is a heteroatom, e.g., oxygen, nitrogen, sulfur, silicon, phosphorus or combinations of one or more thereof. [0046] The term “cycloheteroalkyl” as used herein refers to cycloalkyl radicals in which one or more of the skeletal ring atoms is a heteroatom, e.g., oxygen, nitrogen, sulfur, silicon, phosphorus or combinations of one or more thereof. In some embodiments, the cycloheteroalkyl is pyrrolidinyl, piperidinyl, tetrahydrofuranyl, tetrahydropyranyl or morpholinyl. [0047] The term “heteroaryl” as used herein refers to aryl radicals in which one or more of the skeletal ring atoms is a heteroatom, e.g., oxygen, nitrogen, sulfur, silicon, phosphorus or combinations of one or more thereof. In some embodiments, the heteroaryl is pyrrolyl, pyridinyl or furanyl. [0048] The term “substituted” as used herein refers to a radical in which a hydrogen on one or more carbons of the hydrocarbon backbone has been replaced with a substituent. Such substituents can include, for example, halo, hydroxyl, alkoxyl, silyloxy, carbonyl, phosphoryl, amino, amidyl, iminyl, thiol, thioalkyl, sulfonyl and nitro. It will be understood by those skilled in the art that other substituents known in the art may be used. [0049] As discussed above, the present invention contemplates isotopologues of the compounds. Accordingly, in some embodiments, an amount of the compound may be enriched with an unnatural proportion of one or more isotopes. For example, one or more hydrogen atoms of the compound may be enriched with deuterium. Examples of suitable isotopes include, but are not limited to, isotopes of hydrogen such as deuterium and tritium, isotopes of carbon such as ¹³ C and ¹⁴ C, isotopes of nitrogen such as ¹⁴ N and ¹⁵ N, and isotopes of oxygen such as ¹⁶ O, ¹⁷ O and ¹⁸ O. [0050] As used herein “(s)” following a noun means the plural and/or singular forms of the noun. [0051] As used herein the term “and/or” means “and” or “or” or both. [0052] The term “comprising” as used in this specification means “consisting at least in part of”. When interpreting each statement in this specification that includes the term “comprising”, features other than that or those prefaced by the term may also be present. Related terms such as “comprise” and “comprises” are to be interpreted in the same manner. [0053] It is intended that reference to a range of numbers disclosed herein (for example, 1 to 10) also incorporates reference to all rational numbers within that range (for example, 1, 1.1, 2, 3, 3.9, 4, 5, 6, 6.5, 7, 8, 9 and 10) and also any range of rational numbers within that range (for example, 2 to 8, 1.5 to 5.5 and 3.1 to 4.7) and, therefore, all sub-ranges of all ranges expressly disclosed herein are hereby expressly disclosed. These are only examples of what is specifically intended and all possible combinations of numerical values between the lowest value and the highest value enumerated are to be considered to be expressly stated in this application in a similar manner. [0054] Although the present invention is broadly as defined above, those persons skilled in the art will appreciate that the invention is not limited thereto and that the invention also includes embodiments of which the following description gives examples. BRIEF DESCRIPTION OF THE DRAWINGS [0055] The invention will now be described with reference to the Figures in which: [0056] Figure 1 shows an ORTEP representation (ellipsoid 30% probability) of [HC{(Me)CN(2,6- ⁱ Pr ₂ C ₆ H ₃ )} ₂ YbN(SiMe ₃ ) ₂ ] (1); [0057] Figure 2 shows an ORTEP representation (ellipsoid 30% probability) of [HC{(Me)CN(2,6- ⁱ Pr ₂ C ₆ H ₃ )} ₂ YbH] ₂ (2); [0058] Figure 3 shows an ORTEP representation (ellipsoid 30% probability) of [HC{(Me)CN(2,6- ⁱ Pr ₂ C ₆ H ₃ )} ₂ Yb(C ₂ H ₅ )] ₂ (3); [0059] Figure 4 shows an ORTEP representation (ellipsoid 30% probability) of [HC{(Me)CN(2,6- ⁱ Pr ₂ C ₆ H ₃ )} ₂ Yb(C ₂ H ₅ )] ₂ (4) [0060] Figure 5 shows a 1) ¹ H NMR spectrum of predominantly compound 3 in C ₆ D ₆ ; 2) same sample after ca. 3 hours at room temperature; 3) same sample after ca. 12 hours; 4) same sample after ca. 1 day at room temperature; [0061] Figure 6 shows a 1) ¹ H NMR spectrum of predominantly compound 4 in C ₆ D ₆ ; 2) same sample after ca. 3 hours at room temperature; 3) same sample after ca. 12 hours; 4) same sample after ca. 1 day at room temperature. DETAILED DESCRIPTION OF THE INVENTION [0062] The inventors have developed a process for hydroarylation of an olefin to obtain an alkylated benzene product. The inventors have established that, surprisingly, the hydroarylation reaction may be performed in the presence of a lanthanide complex. Without wishing to be bound by theory, it is believed the lanthanide complex can sequester the olefin to form a lanthanide alkyl complex. This complex may then react with an aryl substrate to provide the alkylated benzene product. Advantageously, the reaction conditions may be selective for anti- Markovnikov alkylated benzene products. [0063] The hydroarylation process may be performed as a one-pot process or as a sequence of reactions. Accordingly, in some embodiments, the hydroarylation process involves reacting a compound of formula II with a compound of formula III in the presence of a lanthanide complex to provide a compound of formula I. In some other embodiments, the compound of formula II is first reacted with the lanthanide complex to provide an intermediate compound of formula VII. The intermediate is then reacted with a compound of formula III to provide the compound of formula I. The intermediate compound of formula VII is optionally isolated before subsequent reaction with the compound of formula III. Alternatively, the intermediate compound of formula VII may be formed in situ and subsequently reacted with the compound of formula III without being isolated. [0064] In some embodiments, the olefin starting material is a compound of formula II, wherein R ₁ , R ₂ , R ₃ and R ₄ are independently selected from the group consisting of H, alkyl, alkenyl, alkynyl, cycloalkyl, cycloalkenyl, heteroalkyl, heteroalkenyl, cycloheteroalkyl and cycloheteroalkenyl, provided that at least one of R ₃ and R ₄ is H and wherein each of said alkyl, alkenyl, alkynyl, cycloalkyl, cycloalkenyl, heteroalkyl, heteroalkenyl, cycloheteroalkyl and cycloheteroalkenyl is optionally substituted. In some embodiments, R ₁ , R ₂ , R ₃ and R ₄ are independently selected from the group consisting of H, C _1-10 alkyl, C _2-10 alkenyl, C _2-10 alkynyl, C _{3- 10} cycloalkyl, C _3-10 cycloalkenyl, C _1-10 heteroalkyl, C _2-10 heteroalkenyl, C _3-10 cycloheteroalkyl and C _3-10 cycloheteroalkenyl. In some embodiments, R ₁ , R ₂ , R ₃ and R ₄ are independently selected from the group consisting of H, C _1-6 alkyl, C _2-6 alkenyl, C _2-6 alkynyl, C _3-6 cycloalkyl, C _{3- 6} cycloalkenyl, C _1-6 heteroalkyl, C _2-6 heteroalkenyl, C _3-6 cycloheteroalkyl and C _3-6 cycloheteroalkenyl. Preferably, R ₁ , R ₂ , R ₃ and R ₄ are independently selected from the group consisting of H, unsubstituted C _1-10 alkyl and unsubstituted C _2-10 alkenyl. More preferably, R ₁ , R ₂ , R ₃ and R ₄ are independently selected from the group consisting of H and unsubstituted C _1-6 alkyl. In some embodiments, R ₃ and R ₄ are H. In some embodiments, R ₂ , R ₃ and R ₄ are H. Examples of the compound of formula II include, but are not limited to, ethylene, propylene, 1-butene, 2- butene, 1,3-butadiene, isobutylene and 1-hexene. [0065] In some embodiments, the compound of formula II is a gas, e.g. ethylene or propylene. In these embodiments, the reaction may be performed by exposing a reaction mixture comprising the compound of formula III and the lanthanide complex or a reaction mixture comprising the intermediate compound of formula VII to an atmosphere comprising the olefin gas. For example, the reaction may be performed under an atmosphere comprising the olefin gas at a pressure in the range of about 0.5 to about 2 atmospheres, such as about 0.5, 1, 1.5 or 2 atmospheres. [0066] In some embodiments, the aryl starting material is a compound of formula III, wherein R ₅ , R ₆ , R ₇ , R ₈ and R ₉ are independently selected from the group consisting of H, alkyl, alkenyl, alkynyl, cycloalkyl, cycloalkenyl, heteroalkyl, heteroalkenyl, cycloheteroalkyl and cycloheteroalkenyl, wherein each of said alkyl, alkenyl, alkynyl, cycloalkyl, cycloalkenyl, heteroalkyl, heteroalkenyl, cycloheteroalkyl and cycloheteroalkenyl may be optionally substituted. In some embodiments, R ₅ , R ₆ , R ₇ , R ₈ and R ₉ are independently selected from the group consisting of H, C _1-10 alkyl, C _2-10 alkenyl, C _2-10 alkynyl, C _3-10 cycloalkyl, C _3-10 cycloalkenyl, C _1-10 heteroalkyl, C _2-10 heteroalkenyl, C _3-10 cycloheteroalkyl and C _3-10 cycloheteroalkenyl. In some embodiments, R5, R6, R7, R8 and R9 are independently selected from the group consisting of H, C _1-6 alkyl, C _2-6 alkenyl, C _2-6 alkynyl, C _3-6 cycloalkyl, C _3-6 cycloalkenyl, C _1-6 heteroalkyl, C _{2- 6} heteroalkenyl, C _3-10 cycloheteroalkyl and C _3-10 cycloheteroalkenyl. Preferably, R ₅ , R ₆ , R ₇ , R ₈ and R ₉ are independently selected from the group consisting of H and unsubstituted C _1-10 alkyl. More preferably, R ₅ , R ₆ , R ₇ , R ₈ and R ₉ are independently selected from the group consisting of H and unsubstituted C _1-6 alkyl. In some embodiments, R ₅ and R ₉ are unsubstituted C _1-6 alkyl, e.g. iso- propyl or iso-pentyl, and R ₆ , R ₇ and R ₈ are H. In some other embodiments, R ₅ , R ₆ , R ₇ , R ₈ and R ₉ are H. [0067] In some embodiments, the hydroarylation may be performed as an intramolecular reaction to obtain a bicyclic product. In these embodiments, R ₁ and R ₅ taken together form –X- (CH2)n– wherein X is C, O, NRa or S, Ra is H or alkyl, and n is 0, 1, 2, 3 or 4, and R2, R3, R4 R6, R ₇ , R ₈ and R ₉ are as defined above. Preferably, n is 0 or 1. [0068] For example, the hydroarylation reaction may comprise the intramolecular reaction of a compound of formula IIIa IIIa in the presence of a lanthanide complex, to obtain a compound of formula Ia [0069] The hydroarylation reaction is preferably performed in an aprotic solvent, more preferably a non-polar solvent. Suitable solvents include, but are not limited to, pentane, hexane, cyclohexane, benzene, alkylated benzene such as toluene, xylene and mesitylene, haloaryl solvents and nitrobenzene. Generally, solvents that may coordinate with the lanthanide complex should be avoid, e.g. THF, ethers, heteroaryl solvents (e.g. pyridine) and heterocyclic solvents (e.g lutidine). Without wishing to be bound by theory, it is believed that solvents that may coordinate with the lanthanide complex may reduce the reactivity of the complex in the hydroarylation reaction. [0070] The hydroarylation reaction may be performed at a temperature from room temperature to about 150°C. In some embodiments, the reaction may be performed at room temperature. In some embodiments, the reaction is performed at about 20°C, about 25°C, about 30°C, about 40°C, about 50°C, about 60°C, about 70°C, about 80°C, about 90°C or about 100°C, preferably about 25°C, about 30°C or about 40°C. The reaction may be performed for a period from 0 to about 48 hours. In some embodiments, the reaction is performed for about 16 hours to about 24 hours. Temperatures and reaction times outside these ranges may, however, still be useful. [0071] The lanthanide complex is a metallic complex comprising a lanthanide metal and one or more ligands. Suitable lanthanide metals include, but are not limited to, ytterbium, europium, thulium, dysprosium and samarium. Preferably, the lanthanide metal is ytterbium. In some embodiments, the lanthanide metal is complexed to a bidentate ligand, such as a β-diiminate bidentate ligand. Preferably the lanthanide complex is a lanthanide (II) complex, and more preferably a lanthanide (II) hydride. The lanthanide may exist in the form of a monomer, a dimer or a combination thereof. Preferably, the lanthanide complex is a compound of formula IV wherein M is selected from the group consisting of ytterbium, europium, thulium, dysprosium and samarium; R ₁₀ and R ₁₄ are the same and are selected from the group consisting of aryl and heteroaryl; R ₁₁ and R ₁₃ are the same and are selected from the group consisting of H, alkyl, alkenyl, alkynyl, cycloalkyl, cycloalkenyl, heteroalkyl, heteroalkenyl, cycloheteroalkyl and cycloheteroalkenyl; R ₁₂ is H; and wherein each of said aryl, heteroaryl, alkyl, alkenyl, alkynyl, cycloalkyl, cycloalkenyl, heteroalkyl, heteroalkenyl, cycloheteroalkyl and cycloheteroalkenyl is optionally substituted. [0072] In some embodiments, R ₁₀ and R ₁₄ are substituted aryl, e.g. aryl substituted with one or more alkyl. In some embodiments, R ₁₀ and R ₁₄ are aryl substituted with one or more C _1-6 alkyl, e.g. 2,6-diisopropylphenyl or 2,6-diisopentylphenyl. [0073] In some embodiments, R ₁₁ and R ₁₃ are H. In some other embodiments, R ₁₁ and R ₁₃ are selected from the group consisting of C _1-6 alkyl, CF ₃ and aryl. Preferably, R ₁₁ and R ₁₃ are C _{1- 6} alkyl, e.g. methyl, ethyl or propyl. More preferably, R ₁₁ and R ₁₃ are methyl. [0074] In some embodiments, the lanthanide complex is a compound of formula IV, wherein R ₁₀ and R ₁₄ are 2,6-diisopropylphenyl, R ₁₁ and R ₁₃ are methyl; and R ₁₂ is H. In some embodiments, the lanthanide complex is a compound of formula IV, wherein R ₁₀ and R ₁₄ are 2,6- diisopentylphenyl, R ₁₁ and R ₁₃ are methyl; and R ₁₂ is H. [0075] In some embodiments, the lanthanide complex is a monomeric compound of formula IVb

wherein R ₁₀ , R ₁₁ , R ₁₂ , R ₁₃ , R ₁₄ and M are as defined above. [0076] In some embodiments, the lanthanide complex is a compound of formula IVc [0077] In some embodiments, the lanthanide complex is used in a catalytic amount. For example, the catalyst loading range of the lanthanide complex may be about 0.01 to about 40 mol%. In some embodiments, the lanthanide complex is used in an amount of about 0.01, about 0.1, about 1, about 5, about 10, about 15, about 20, about 25, about 30, about 35 or about 40 mol% In some embodiments, the lanthanide complex is used in a stoichiometric amount. [0078] The lanthanide complex and precursors thereof may be prepared by standard techniques known in the art. For example, the lanthanide (II) hydride complex may be prepared by reacting a lanthanide amide precursor with a silane, e.g. PhSiH ₃ . The silane is typically used in excess relative to the lanthanide amide precursor, e.g. 2, 3, 4 or 5 equivalents, preferably 3 equivalents, of silane may be reacted with the lanthanide amide precursor. The reaction is generally performed at room temperature. Temperatures outside this range may, however, still be useful. [0079] A suitable lanthanide amide precursor is a compound of formula V, wherein R ₁₀ , R ₁₁ , R ₁₂ , R ₁₃ , R ₁₄ and R ₁₅ are as defined above. [0080] Preferably, the lanthanide amide precursor and silane are reacted in an aprotic solvent, more preferably a non-polar solvent. Suitable solvents include, but are not limited to, pentane, hexane, cyclohexane, benzene, an alkylated benzene, a haloaryl solvent and nitrobenzene. For the same reasons as mentioned above, it is preferable to avoid solvents such as THF that may coordinate with the lanthanide complex. [0081] The lanthanide amide precursor may be prepared by reacting a suitable lanthanide starting material, e.g. a ytterbium bis(amide) such as [Yb(N[Si(CH ₃ ) ₃ ] ₂ ) ₂ ] ₂ , with a pre-ligand. For example, the compound of formula Va may be prepared by reacting a compound of formula VI VI with a compound having the formula M(N(R ₁₅ ) ₃ ) ₂ ), wherein R ₁₀ , R ₁₁ , R ₁₂ , R ₁₃ , R ₁₄ , R ₁₅ and M are as defined above. As with other reactions described above, the reaction is preferably performed in an aprotic solvent, more preferably a non-polar solvent. Suitable solvents include, but are not limited to, pentane, hexane, cyclohexane, benzene, an alkylated benzene, a haloaryl solvent and nitrobenzene. [0082] Alternatively, the lanthanide (II) hydride complex may be prepared by reacting a lanthanide silyl precursor with an alkene, e.g. 1,4-cyclohexadiene. The alkene is typically used in excess relative to the lanthanide silyl precursor, e.g. 5, 10 or 15 equivalents, preferably 10 equivalents, of alkene may be reacted with the lanthanide silyl precursor. The reaction is generally performed at room temperature. Temperatures outside this range may, however, still be useful. [0083] A suitable lanthanide silyl precursor is a compound of formula Vb, wherein R ₁₀ , R ₁₁ , R ₁₂ , R ₁₃ , R ₁₄ and R ₁₆ are as defined above. [0084] Preferably, the lanthanide silyl precursor and alkene are reacted in an aprotic solvent, more preferably a non-polar solvent. Suitable solvents include, but are not limited to, pentane, hexane, cyclohexane, benzene, an alkylated benzene, a haloaryl solvent and nitrobenzene. For the same reasons as mentioned above, it is preferable to avoid solvents such as THF that may coordinate with the lanthanide complex. [0085] The lanthanide silyl precursor may be prepared by reacting a suitable lanthanide starting material, e.g. a ytterbium halide such as [(HC{(Me)CN(2,6- ⁱ Pent ₂ C ₆ H ₃ )} ₂ )YbI] ₂ , with a silyl reagent such as K[CH(SiMe ₃ ) ₂ ]. For example, the compound of formula Vb may be prepared by reacting a compound of formula VIIIb with K[CH(SiMe ₃ ) ₂ ]. The compound of formula VIIIb may be prepared, for example, by reacting a compound of formula IXb IXb with a suitable metal halide, e.g. YbI ₂ . [0086] Each of the reactions above may be performed in an inert atmosphere, e.g. a moisture free and/or oxygen free atmosphere. For example, each of the reactions above may be performed in a nitrogen or argon atmosphere. In some embodiments, the reaction may be performed in an atmosphere comprising the reagent. For example, the reaction of a compound of formula II and a compound of formula III may be performed in an atmosphere comprising a compound of formula II, e.g. ethylene or propylene. In some embodiments, the reaction of a compound of formula II and a compound of formula III may be performed in an atmosphere consisting essentially of a compound of formula II, e.g. ethylene or propylene. [0087] The following non-limiting examples are provided to illustrate the present invention and in no way limit the scope thereof. EXAMPLES Abbreviations [0088] BDI CH[C(CH ₃ )NDipp] ₂ Dipp 2,6-diisopropylphenyl DiPeP 2,6-diisopentylphenyl EXSY Exchange spectroscopy GC-MS Gas chromatography–mass spectrometry NMR Nuclear magnetic resonance ORTEP Oak Ridge Thermal Ellipsoid Plot THF Tetrahydrofuran Methods and materials [0089] All manipulations were performed under a dry, oxygen-free argon atmosphere using standard Schlenk-line techniques, or in a conventional nitrogen-filled glovebox. Solvents were dried over appropriate drying agents and degassed prior to use. NMR spectra were recorded using a JEOL 500 MHz spectrometer, operating at 500 MHz ( ¹ H) or 126 MHz ( ¹³ C). Spectra being recorded at 298 K (unless stated otherwise) and proton and carbon chemical shifts were referenced internally to residual solvent resonances. Coupling constants are quoted in Hz. Elemental analyses were performed by Elemental Microanalysis Ltd. All other chemicals were purchased from Sigma-Aldrich and used without further purification. GC-MS analyses were carried out using a Shimadzu QP2010-Plus gas chromatograph−mass spectrometer equipped with an AOC-20i auto injector. The samples were taken from the NMR tube and diluted 20:1 with chloroform. GC-MS analyses used helium as the carrier gas. Mass spectra were obtained at 70 eV in positive ion mode, scanning at m/z 40−600 every 0.3 s. The ion source was held at 200 °C, while the MS-transfer line was at 305 °C. Compound identity was determined using both retention time and mass spectral fragmentation pattern. Samples were introduced (1 μL) into a glass split/splitless liner at 270 °C. Separations were performed using a Restek RXI-5Sil-MS column (30 m × 0.25 mm × 0.25 µm) with a 50:1 split injection using constant carrier gas flow (linear velocity 43.4 cm/s; 1.38 mL/min). The initial oven temperature was 50 °C, held for 2 min, after which a temperature ramp of 10 °C/min to 300 °C was used, with a final hold of 5 min (total analysis time 32 min). Phenylsilane (97%), ethene (>99.5%) and propene (>99%) were purchased from Sigma-Aldrich Ltd. and used without further purification. Yb{N(SiMe ₃ ) ₂ } ₂ and [HC{(Me)CN(2,6- ⁱ Pr ₂ C ₆ H ₃ )} ₂ ]H were synthesised by literature procedures (e.g. see Tilly, TD PhD Thesis, University of California, 1982 and Stender M, et al. Journal of the Chemical Society, Dalton Transactions 2001(23): 3465-3469.) Preparation of [HC{(Me)CN(2,6- ⁱ Pr2C6H3)}2YbN(SiMe3)2] (1) [0090] A colourless toluene solution of the pre-ligand BDI ^Dipp H (847 mg, 2.02 mmol) was added to an orange toluene solution of Yb(HMDS) ₂ (1000 mg, 2.02 mmol) in an ampule and sealed. The mixture was heated to 100°C for 16 hours. The solvent was removed in vacuo from the resulting dark red solution to give 1 as an analytically pure dark red crystalline solid in essentially quantitative yields. Deep red crystals suitable for X-ray diffraction analysis were obtained from a saturated toluene solution at –30°C. An ORTEP representation of 1 is shown in Figure 1. ¹ H NMR (500 MHz, C ₆ D ₆ ) δ 7.20-7.00 (m, 6H, ArH), 4.88 (s, 1H, NC(CH ₃ )CH), 3.30 (sept, J = 6.9 Hz, 4H, CH(CH ₃ ) ₂ ), 1.72 (s, 6H, NC(CH ₃ )CH), 1.32 (d, J = 6.9 Hz, 12H, CH(CH ₃ ) ₂ ), 1.28 (d, J = 6.9 Hz, 12H, CH(CH ₃ ) ₂ ), 0.12 (s, 18H, Si(CH ₃ ) ₃ ). ¹³ C NMR (126 MHz, C ₆ D ₆ ) δ 165.65 (NC(CH ₃ )CH), 146.72 (C _ipso ), 141.13 (C _ortho ), 124.95 (C _para ), 124.19 (C _meta ), 91.75 (NC(CH ₃ )CH), 28.59 (CH(CH ₃ ) ₂ ), 25.79, 25.48 (CH(CH ₃ ) ₂ ), 24.74 (NC(CH ₃ )CH), 5.52 (Si(CH ₃ ) ₃ ). Preparation of [HC{(Me)CN(2,6- ⁱ Pr2C6H3)}2YbH]2 (2) [0091] A colourless toluene solution containing 3 equivalents of phenylsilane (129.4 mg, 1.19 mmol) was added to a dark red toluene solution of 1 (300 mg, 0.39mmol) and left to stir at room temperature for 30 minutes which resulted in a black solution. Solvent was removed in vacuo to give the crude product as a thick black paste, which was re-dissolved into hexane to give a black solution. The solution was filtered, concentrated and crystallised at –30°C overnight to give 2 black crystals (179 mg, 77%). An ORTEP representation of 2 is shown in Figure 2. ¹ H NMR (500 MHz, C ₆ D ₆ ) δ 7.93 (t, J = 398 Hz, 1H, YbH), 7.01 (d, J = 7.6 Hz, 4H, ArH), 6.52 (t, J = 7.6 Hz, 2H, ArH), 4.84 (s, 1H, NC(CH ₃ )CH), 3.14 (sept, J = 6.8 Hz, 4H, CH(CH ₃ ) ₂ ), 1.68 (s, 6H, NC(CH ₃ )CH), 1.34 (d, J = 6.8 Hz, 12H, CH(CH ₃ ) ₂ ), 1.12 (d, J = 6.8 Hz, 12H, CH(CH ₃ ) ₂ ). ¹³ C{ ¹ H} NMR (126 MHz, C ₆ D ₆ ) δ 163.14 (NC(CH ₃ )CH), 147.04 (C _ipso ), 143.59 (C _ortho ), 123.52 (C _para ), 123.13 (C _meta ), 94.95 (NC(CH ₃ )CH), 29.08 (CH(CH ₃ ) ₂ ), 25.15, 25.07 (CH(CH ₃ ) ₂ ), 24.15 (NC(CH3)CH). Preparation of [HC{(Me)CN(2,6- ⁱ Pr ₂ C ₆ H ₃ )} ₂ Yb(C ₂ H ₅ )] ₂ (3) [0092] In a J Youngs tap NMR tube, a black solution of 2 (74 mg, 0.06 mmol) in hexane was degassed via three freeze-pump-thaw cycles before being exposed to 1 atmosphere of ethylene and left at room temperature for 16 hours. The hexane solution was decanted from the resulting black–brown crystalline solid which was dried in vacuo to give 3 (31 mg, 42% isolated yield) as a black solid. Black–brown crystals suitable for X-ray diffraction analysis were obtained from a 100:1 hexane:toluene solution of 3 at –30°C. An ORTEP representation of 3 is shown in Figure 3. ¹ H NMR (500 MHz, C ₆ D ₆ ) δ 7.15 - 7.11 (m, 6H, ArH), 4.66 (s, 1H, NC(CH ₃ )CH), 3.19 (sept, J = 6.9 Hz, 4H, CH(CH ₃ )), 1.52 (s, 6H, NC(CH ₃ )CH), 1.38 (t, J = 8.1 Hz, 3H, YbCH ₂ CH ₃ ) 1.17 (d, J = 6.9 Hz, 12H, CH(CH3)2), 1.03 (d, J = 6.9 Hz, 12H, CH(CH3)2), -0.37 (q, J = 8.1 Hz, 2H, YbCH ₂ CH ₃ ). ¹³ C{ ¹ H} NMR (126 MHz, C ₆ D ₆ ) δ165.63 (NC(CH ₃ )CH), 145.27 (C _ipso ), 142.07 (C _ortho ), 124.77 (C _para ), 124.03 (C _meta ), 94.54 (NC(CH ₃ )CH), 28.40 (CH(CH ₃ ) ₂ ), 24.89, 24.77 (CH(CH ₃ ) ₂ ), 24.50 (NC(CH ₃ )CH), 15.80 (YbCH ₂ CH ₃ ), 14.35 (YbCH ₂ CH ₃ ). Preparation of [HC{(Me)CN(2,6- ⁱ Pr ₂ C ₆ H ₃ )} ₂ Yb(C ₃ H ₇ )] ₂ (4) [0093] In a J Youngs tap NMR tube, a black solution of 2 (49 mg, 0.0414 mmol) in hexane was degassed via three freeze-pump-thaw cycles before being exposed to 1 atmosphere of propylene and left at room temperature for 16 hours. The hexane solution was decanted from the resulting black–brown crystalline solid which was dried in vacuo to give 4 (34 mg, 64% isolated yield) as a black solid. Black–brown crystals suitable for X-ray diffraction analysis were obtained from a 100:1 hexane:toluene solution of 4 at –30°C. An ORTEP representation of 4 is shown in Figure 4. ¹ H NMR (500 MHz, C ₆ D ₆ ) δ 7.20 – 7.17 (m, 4H, ArH), 7.14 – 7.11 (m, 2H, ArH), 4.67 (s, 1H, NC(CH ₃ )CH), 3.21 (sept, J = 6.8 Hz, 4H, CH(CH ₃ )), 1.52 (s, 6H, NC(CH ₃ )CH), 1.18 (d, J = 6.8 Hz, 12H, CH(CH ₃ ) ₂ ), 1.14 (m, 5H, YbCH ₂ CH ₂ CH ₃ overlapping YbCH ₂ CH ₂ CH ₃ ), 1.05 (d, J = 6.8 Hz, 12H, CH(CH ₃ ) ₂ ), -0.29 (t, J = 8.8 Hz, 2H, YbCH ₂ CH ₂ CH ₃ ). ¹³ C NMR (126 MHz, C ₆ D ₆ ) δ165.71 (NC(CH ₃ )CH), 145.42 (C _ipso ), 142.02 (C _ortho ), 124.76 (C _para ), 124.00 (C _meta ), 94.31 (NC(CH ₃ )CH), 28.43 (CH(CH ₃ ) ₂ ), 24.89, 24.82 (CH(CH ₃ ) ₂ ), 24.50 (NC(CH ₃ )CH), 23.01 (YbCH ₂ CH ₂ CH ₃ ). Preparation and characterisation of 3 and 4 [0094] The synthesis of the low-coordinate ytterbium n-alkyls, 3 and 4, is illustrated in Scheme 1. The reaction of the solvent-free ytterbium bis(amide), [Yb(N[Si(CH ₃ ) ₃ ] ₂ ) ₂ ] ₂ , with the β- diimine pro-ligand, BDI ^Dipp -H, refluxed in toluene for 12 hours cleanly generated the ytterbium amide, [BDI ^Dipp Yb(N[Si(CH ₃ ) ₃ ] ₂ )] ₄ (1) in essentially quantitative yield as a red crystalline solid. The solid state structure of 1 is shown in Figure 1. Addition of phenylsilane to 1 provides the ytterbium hydride, [BDI ^Dipp YbH] ₂ 2, in good yields (>80%) as an extremely air- and moisture- sensitive black crystalline material. In the solid state, 2 adopts a dimeric structure with two μ ² - hydride ligands bridging the Yb(II) centres (Figure 2). Each Yb(II) centre binds to the β- diketiminato-N atoms and interacts in a η ⁶ -coordination mode (Yb1-Ar _cent 2.7099(9) Å) with a Dipp substituent of a second BDI ^Dipp YbH unit of the dimer. The η ⁶ -coordination of 2 is significantly weaker than in other ytterbium(II) complexes with a Yb(II) κ ¹ -N,η ⁶ -Dipp chelate (2.424 – 2.520 Å) (e.g. see Crimmin, MR et al. New J. Chem. 2010, 34(8), 1572-1578; Trifonov, AA et al. Russian Chem. Bull. 2018, 67(3), 455-460; and Heitmann, D et al. Dalton Trans. 2007(2), 187-189) The weaker η ⁶ -coordination of 2 suggests that the solid-state structure may not be retained in solution. Scheme 1: Synthesis of the ytterbium hydride (2) and alkyl derivatives (3 and 4). [0095] When isolated samples of 2 were dissolved and analysed by multinuclear NMR spectroscopy, two distinct species are discriminated in the solution state in a 25:1 ratio. The major species displayed a peak at 7.52 ppm, ¹ J _Yb-H = 398 Hz, which was attributed to the hydride resonance. The minor component in solution displayed a second hydride resonance centred at 9.92 ppm. This resonance is consistent with other dimeric ytterbium hydrides with only the hydrido ligands μ ² -bridging two Yb(II) centres and with no arene interactions. Therefore, the minor product in solution was tentatively assigned as 2'. Exchange spectroscopy (EXSY) NMR experiments demonstrated that 2 and 2' are in equilibrium at room temperature. Although, attempts to quantify this process by variable temperature ¹ H NMR studies were hindered, it is believed, by the competitive redistribution to the homoleptic ytterbium complex, [(BDI ^Dipp ) ₂ Yb], and facile Yb-H/D exchange with the deuterobenzene solvent. In this latter regard, the signal associated with the hydride ligand of 2 was observed to decrease if left in benzene-d ₆ for extended periods of time (>6h), concomitant with the formation of a new signal in the ² H NMR spectrum centred at 7.92 ppm, which was assigned as the ytterbium deuteride [BDI ^Dipp YbD] ₂ , 2- d. Reaction of compounds 3 and 4 with benzene-d ₆ [0096] Compounds 3 and 4 were reacted with benzene-d ₆ as shown in Scheme 2. Scheme 2: Reaction of ytterbium alkyls with benzene. [0097] In a J Youngs tap NMR tube, a brown-black solution of 3 (20 mg, 0.016 mmol) in benzene-d ₆ was monitored by NMR spectroscopy at room temperature for ca. 1 day (Figure 5). The volatile ethylbenzene-d ₅ was vacuum transferred by trap to trap distillation. This reaction resulted in the stoichiometric production of n-ethylbenzene-d ₅ identified by ¹ H NMR spectroscopy and GC-MS as the sole organic product of the reaction. [0098] The same experiment was repeated with compound 4. In a J Youngs tap NMR tube, a brown-black solution of 4 (20 mg, 0.016 mmol) in benzene-d ₆ was monitored by NMR spectroscopy at room temperature for ca. 1 day (Figure 6). The volatile n-propylbenzene-d ₅ was vacuum transferred by trap to trap distillation. This reaction resulted in the stoichiometric production of n-propylbenzene-d ₅ identified by ¹ H NMR spectroscopy and GC-MS as the sole organic product of the reaction. Preparation of [(HC{(Me)CN(2,6- ⁱ Pent2C6H3)}2)YbI]2 (5) [0099] A pale brown Et2O solution of (BDI ^Dipep )K (1000 mg, 1.76 mmol) was added to a scintillation vial containing a pale-yellow suspension of YbI ₂ (750.2 mg, 1.76 mmol) in Et ₂ O and left to stir at room temperature for 48 hours. The mixture was filtered through celite, and the solvent removed in vacuo from the resultant dark red solution to provide the crude product (5) as a red solid. Deep red crystals suitable for X-ray diffraction analysis were obtained from a saturated pentane solution of 5 at room temperature. ¹ H NMR (500 MHz, C ₆ D ₆ ) δ 7.10 (m, 6H, ArH), 4.79 (s, 1H, NC(CH ₃ )CH), 2.77 (m, 4H, CH(CH ₂ CH ₃ ) ₂ ), 1.74 – 1.16 (m, 16H, CH(CH ₂ CH ₃ ) ₂ ), 1.73 (s, 12H, NC(CH ₃ )CH), 1.03 (t, J = 6.8 Hz, 12H, CH(CH ₂ CH ₃ ) ₂ ), 0.83 (t, J = 6.8 Hz, 12H, CH(CH ₂ CH ₃ ) ₂ ). ¹³ C NMR (126 MHz, C ₆ D ₆ ) δ 165.6 (NC(CH ₃ )CH), 148.9 (C _ipso ), 138.3 (C _ortho ), 125.7 (C _para ), 123.6 (C _meta ), 92.3 (NC(CH ₃ )CH), 41.0 (NC(CH ₃ )CH), 27.9, 26.7, 25.6, 25.4 (CH(CH2CH3)2), 11.9, 11.4 (CH(CH2CH3)2). Preparation of [(HC{(Me)CN(2,6- ⁱ Pent ₂ C ₆ H ₃ )} ₂ )YbCH(SiMe ₃ ) ₂ ] ₂ (6) [00100] A red toluene solution of 5 (300 mg, 0.18 mmol) was added to a scintillation vial containing a toluene slurry of K[CH(SiMe ₃ ) ₂ ] (68.1 mg, 0.34 mmol) and was left to stir for ca. 48 hours at room temperature. The mixture was filtered through celite, and the solvent removed in vacuo from the resultant dark brown-red solution to generate the crude product (6) as a red solid. Deep red crystals suitable for X-ray diffraction analysis were obtained from a saturated hexane solution of 6 at room temperature. ¹ H NMR (500 MHz, C ₆ D ₆ ) δ 7.10 – 7.08 (m, 2H, ArH), 7.08 – 7.02 (m, 4H, ArH), 4.80 (s, 1H, NC(CH ₃ )CH), 2.75 – 2.70 (m, 4H, CH(CH ₂ CH ₃ ) ₂ ), 1.69 – 1.62 (m, 16H, CH(CH ₂ CH ₃ ) ₂ ), 1.68 (s, 12H, NC(CH ₃ )CH), 1.05 (t, J = 6.8 Hz, 12H, CH(CH2CH3)2), 0.83 (t, J = 6.8 Hz, 12H, CH(CH2CH3)2), 0.01 (s, 18H, CH(Si(CH3)3)2), – 1.82 (s, 1H, CH(Si(CH ₃ ) ₃ ) ₂ ). ¹³ C NMR (126 MHz, C ₆ D ₆ ) δ 165.2 (NC(CH ₃ )CH), 146.5 (C _ipso ), 138.5 (C _ortho ), 126.4 (C _para ), 124.4 (C _meta ), 94.3 (NC(CH ₃ )CH), 42.7 (NC(CH ₃ )CH), 29.2, 26.4, 25.0 (CH(CH ₂ CH ₃ ) ₂ ), 13.3, 11.8 (CH(CH ₂ CH ₃ ) ₂ ), 5.8 (CH(Si(CH ₃ ) ₃ ) ₂ ). Preparation of [(HC{(Me)CN(2,6- ⁱ Pent ₂ C ₆ H ₃ )} ₂ )YbH] ₂ (7) [00101] A colourless C ₆ D ₆ solution of 1,4-cyclohexadiene (54.3 mg, 0.68 mmol) was added to a red C ₆ D ₆ solution of 6 (63.5 mg, 0.068 mmol) in an NMR tube fitted with a J Youngs tap. The reaction mixture was left to heat overnight at 40°C to generate a brown solution. The volatiles were removed under vacuum to provide the crude product (7) as a brown solid. Brown crystals suitable for X-ray diffraction analysis were obtained from a saturated hexane solution of 7 at –30°C. ¹ H NMR (500 MHz, C ₆ D ₆ ) δ 9.08 (s, 1H, Yb-H), 7.12 – 7.09 (m, 2H, ArH), 7.04 – 7.03 (m, 4H, ArH), 4.74 (s, 1H, NC(CH3)CH), 2.71 – 2.66 (m, 4H, CH(CH2CH3)2), 1.72 – 1.62 (m, 8H, CH(CH ₂ CH ₃ ) ₂ ), 1.62 (s, 12H, NC(CH ₃ )CH), 1.62 – 1.49 (m, 8H, CH(CH ₂ CH ₃ ) ₂ ), 0.87 (bt, 24H, CH(CH ₂ CH ₃ ) ₂ ). ¹³ C NMR (126 MHz, C ₆ D ₆ ) δ 164.2 (NC(CH ₃ )CH), 146.5 (C _ipso ), 139.9 (C _ortho ), 125.1 (C _para ), 123.9 (C _meta ), 94.6 (NC(CH ₃ )CH), 42.3 (NC(CH ₃ )CH), 29.5, 26.6, 25.0 (CH(CH ₂ CH ₃ ) ₂ ), 13.6, 12.1 (CH(CH ₂ CH ₃ ) ₂ ). Catalytic hydrophenylation of ethylene and propylene [00102] The ultimate by-product of the reactions described above is the regeneration of the ytterbium(II) hydride (2/2'). This by-product is the starting material for the synthesis of the ytterbium(II) n-alkyl compounds, indicating that this transformation may be applied to a catalytic regime. The reaction was studied by reacting one atmosphere of ethylene in the presence of 0.026 mol/L of ytterbium hydride 2 dissolved in degassed benzene-d6 at room temperature. The reaction was monitored by ¹ H NMR spectroscopy over the course of 5 days. The resultant ¹ H NMR spectra demonstrated a slow but steady decrease in ethylene, concomitant with the growth of the diagnostic n-ethyl-benzene methylene signal centred at 2.45 ppm. The reaction was repeated with a 20 mg sample of compound 2 in 0.6 mL of benzene-d ₆ under 1 atmosphere of ethylene heated to 40°C and monitored for 24 hours and 44 hours with spectra recorded every hour. These spectra also showed the growth of the diagnostic n-ethyl-benzene methylene signal centred at 2.45 ppm. [00103] Analysis of the reaction solution demonstrated the presence of ethylbenzene and a small proportion of n-butylbenzene in a 100:5 ratio. The reaction solution also contained an insoluble precipitate, which was identified as polyethene by analysis with Fourier transform infrared (FTIR) spectroscopy. [00104] In an attempt to control the selectivity of this transformation, the reaction was repeated at various pressures of ethylene, specifically 0.5, 1 and 1.5 atmospheres. No variation in product ratio was observed. Advantageously, the data from these experiments demonstrated that 2 is capable of catalysing the hydroarylation of ethylene with benzene. [00105] The reaction of one atmosphere of propylene in the presence of 0.026 mol/L of ytterbium hydride 2 dissolved in degassed benzene-d ₆ was, therefore, monitored by ¹ H NMR spectroscopy at room temperature over the course of 8 days. This reaction proved to be selective for the production of n-propyl-benzene as the sole organic product of the reaction as identified by multinuclear NMR spectroscopy and GC-MS analysis. Crystallography [00106] Compounds Yb{N(SiMe ₃ ) ₂ } ₂ , 1, 2, 3 and 4 were collected on an Agilent SuperNova diffractometer fitted with an EOS S2 detector. All datasets were collected using CuKα radiation (λ = 1.54184 Å). The crystal structure of compound 1 had unresolvable solvent electron density, which was accounted for using SQUEEZE ³ and produced infinite void channels along the c-axis. Crystals of 3 were non-merohedrally twinned; the resulting crystal structure was solved using the major component and finally refined using both twin components (HKLF5 refinement). Conclusion [00107] In summary, described herein is the synthesis of a lanthanide (II) hydride complex, which was shown to sequester an olefin to give the corresponding alkyl lanthanide complex. The n-alkyl lanthanide (II) complexes disclosed herein can mediate the hydroarylation of olefins with an aryl group. The reactivity of these lanthanide (II) alkyls diverges from all previously reported lanthanide alkyl complexes, which exclusively undergo σ-bond metathesis reactions with aryl C-H bonds, yielding lanthanide aryl and alkane products. The by-product from the stoichiometric reactions of the n-alkyls lanthanide (II) complexes with benzene is the regeneration of the lanthanide hydride demonstrating the reactivity may be applied to a catalytic regime. More generally, nucleophilic lanthanide alkyls have been shown to catalyse the hydroarylation of unactivated olefins with arene C-H bonds to provide the linear alkylated arene products. [00108] It is not the intention to limit the scope of the invention to the abovementioned examples only. As would be appreciated by a skilled person in the art, many variations are possible without departing from the scope of the invention as set out in the accompanying claims.