2. Description of the Related Art The legacy of fifty years of nuclear processing for weapons and power production is a staggering environmental problem. In the United States alone there are more than 100 contaminated installations in 36 states and territories, and similar situations exist in other countries. The cost of cleanup using current technologies have been estimated to be in excess of $100 billion. Defense waste accounts for 95% of the volume of the total U. S. radioactive waste, but 90% of the activity is contained in spent commercial nuclear fuel.

The most serious challenge in terms of environmental restoration is provided by the tank wastes at the Department of Energy (DOE) Hanford site. Most of this waste is stored in about 60 undergound tanks each holding a million gallons of alkaline slurry.

The chemical compositions of the tank wastes are highly complex and are in many cases unknown in detail, since waste streams from several fuel-processing operations were combined, together with residues from many other secondary processes. Acidic waste streams were treated with sodium hydroxide to minimize tank corrosion, making the waste highly concentrated in non-radioactive sodium salts. The tanks are known to contain significant concentrations of about 50 elements and a range of organic species (solvents and sequestrants). The radioactive elements present that are of most concern from the viewpoint of toxicity levels are Sr, Tc, Cs, U, Pu and other transuranics. Other radioactive species include Y, I, Ce, Pm and Eu. All of the components of the wastes are distributed between insoluble sludge, a high ionic strength aqueous supernatant, and a soluble salt case crystallized from the supernatant.

Three phases of processing have generally been considered for waste treatment : (a) pretreatment of raw tank waste for subsequent separations ; (b) physical or chemical separations ; (c) solidification (usually vitrification) of high level waste (HLW) for subsequent deep geologic storage. Although several potential processes have been described, serious difficulties and limitations remain. These include the distribution of radioactive components between the supernatant, salt cake, and sludge ; the difficulties inherent in handling possible colloidal dispersions of sludge particles ; and the incomplete characterization of the chemical species present. Thus, although it might be expected that lanthanide (Ln) and actinide (An) species would be present in the insoluble sludge, since the solutions are highly alkaline, the presence of organic species including sequestrants such as EDTA and nitrilotriacetic acid in some of the tanks may result in partial solubilization.

As mentioned above, Tc (technetium) is a radioactive element which is particularly toxic and often found in tank wastes from nuclear weapons development. It is estimated that 1. 8 metric tons of Tc are present in tank wastes at the Hanford, WA site alone.

Owing to the volatility and leachability of Tc (half-life of 99Tc is approx. 200, 000 yrs) removal and storage is not possible by the vitrification processes in use for all other radioactive components of nuclear waste.

Although TcO2 has very low water solubility and a high Tc content, it can be difficult to separate from solution and is difficult to handle in dry form. The low solubility of Tc02 in water increases dramatically in the presence of oxygen as a result of facile oxidation to the soluble Tc04-anion. At high temperatures used in vitrification procedures, Tc02 disproportionates into metallic Tc and volatile Tc207. Prior methods for removing Tc from waste include : Luo (U. S. Patent No. 5, 994, 609) teaches a method for treating liquid hazardous waste containing anionic radioactive or heavy metal materials by binding the hazardous waste to hydroxyapatite powder ; drying the hydroxyapatite powder ; and cold or hot pressing the hydroxyapatite powder into a solid mass for storage or disposal.

Elfline (U. S. Patent No. 4, 764, 281) teaches a method for treating radioactive metal-containing liquid by contacting the radioactive metal-containing liquid with a water-in-soluble carboxylated cellulose to separate the heavy metals from the liquid.

Leitnaker et al (U. S. Patent No. 5, 885, 326) teaches a method for removing technetium from iron, and other metals. This method comprises adding manganese to the technetium-contaminated metal ; melting the mixture to produce a homogenous distribution of manganese and technetium ; and adding sulfur to the molten homogenous mixture to precipitate manganese and technetium as a sulfide slag.

Pal et al (U. S. Patent No. 5, 994, 608) teaches a method of immobilizing radioactive metals in waste which comprises mixing the waste with a reagent to produce a first mixture ; adding a phosphate compound to the first mixture to produce a second mixture ; and curing the second mixture to form a non-leachable solid material.

Snyder et al (U. S. Patent No. 5, 954, 936) teaches a method for removing technetium complexes from liquid by adjusting the pH of the liquid to greater than 2 and directing the solution into an integrated resin and electrochemical plating device.

Saraceno et al (U. S. Patent No. 5, 826, 163) teaches a method for removing technetium from uranium hexafluoride method-involves contacting the uranium hexafluoride with a metal fluoride for a period of time sufficient for the technetium to bind to the metal fluoride. Complexes known as polyoxometalates (POMs, heteropolyanions) form a class that is of unsurpassed structural and electronic variety and versatility in inorganic and organometallic chemistry. These substances are discrete molecular-ionic species with the general formula [XxMmOyin- (m>x), where the addenda atoms, M. typically are VV, NbV, More, WVI, and the heteroatoms, X, can be one or more of about 70 other elements in a variety of oxidation states. Salts and free acids of POM anions are water-and organic-solvent-soluble and generally have high thermal stabilities (about 500 duc). The anions may contain 30 or more metal atoms, and the salts frequently have molecular weights exceeding 10, 000.

POMs have many important applications : they are industrial acid-and redox catalysts (propene hydration, oxidation of methacrolein, etc.) And they find use in analytical and clinical chemistry, medicine (antiviral and antitumoral agents, electron- dense imaging agents), materials science (electronic and protonic conductors), and as selective and high-capacity oxidants and reductants. Their properties and structures are well suited to examining and modeling theories and mechanisms in areas as diverse as electron transfer, mixed valence chemistry, topology, thost-guest and cryptate chemistry, self-assembly and template processes, small molecule activation, polymetallic centers in bioinorganic chemistry, cooperative magnetic behavior, surface science, heterogeneous catalysis, geology, etc.

Polyoxometalates are most commonly formed in aqueous or polar nonaqueous solutions by acid-base condensation-addition processes, e. g., 12 Mo042-+ HP042-+ 23 H+--> [Pmol2040] 3~ + 12 H2O or by hydrothermal methods. The resulting anions are isolated as salts or free acids and are stable in solution over a pH range that is characteristic of the particular anion. The structures are based most often on arrangements of edge-and corner-shared MO6 octahedra, and, for reasons of clarify, are frequently illustrated in this fashion.

Currently, more than 1000 wholly inorganic heteropolyanions of molybdenum, tungsten, and vanadium are known, but the demonstrated possibility of attaching organic and organometallic groups, or incorporating alkoxides, and of partial substitution of 02' by F-, 02-, 022-, S2-, etc., extends the field considerably. As a class, POMs are remarkably stable against hydrolysis and oxidation/reduction. Although the above equation, and its analogues for other POMs, is reversible, the pH at which hydrolysis becomes significant is very much dependent upon the composition and charge of the polyanion. Some species are kinetically inert at pH 12 or higher. Since the component elements are in high oxidation states, POMs are unaffected by strong oxidants. Conversely, reduction of many kinds of POMS leads to a huge class of mixed-valen'ceheteropoly blues"and'-browns", -polyanions that retain the overall molecular structures of the original oxidized species.

The reduced anions show enhanced resistance to alkaline decomposition. It is this feature an ability to absorb large numbers of electrons (up-to two per metal atom) without molecular decomposition-which must partly be responsible for the stability of POMs against ionizing radiation.

Polyoxometalates have been previously used for separations of radionuclides via precipitation or complex formation. For example, the molybdophosphate anion, [PMO12O40] 3, has been used for removal of 137Cs, either by precipitation as the tricesium salt, or by ion exchange using the insoluble ammonium or potassium salts. The success of ammonium molybdophosphate as an ion exchange material is a consequence of the zeolitic-type crystal structure observed for this and many other polyoxometalate salts, namely a lattice of large quasi-spherical polyanions interspersed with weakly-bound water molecules and countercations.

Many types of polyoxometalate complexes of lanthanides or actinides are known.

Such cations demand high coordination numbers (CN=8-12) and can be incorporated into polyoxometalates in three ways : (1) as a central atom in a heteropolyanion ; (2) as a complex with polyoxoanion ligands ; or (3) as a cryptate.

The present inventors found that Tc compounds in the form of monovalent Tc cations bind to the surfaces of nucleophilic polyoxometalate anions to form stable, insoluble, and non-volatile products. Given this ability ofnucleophilic polyoxometalate anions, the present inventors developed a method of separating and concentrating technetium from tank wastes. It is known that radioactive isotopes, such as technetium, constitute less-than one part in 104 of the mass present in most tanks, so that if efficient separations can be devised, the total volume of HLW to be stored is reduced to more manageable quantities. Proposed separations are based on binding to inorganic polyoxometalate complexes. An enormous variety of such complexes is known, and structures can be designed to be selective for certain cations based on size and coordination preferences. In view of the high and varied salt content of tank wastes, high selectivity for the radioactive components is a major advantage. Other advantages of polyoxometalates accrue from their high stabilities in aqueous and non-aqueous media, and in the solid state, and their conversion to possible alternative waste forms for isolation and storage.

As discussed above, the isolation of technetium (99Tc) from the environment is important since it is highly mobile in groundwater. Unfortunately, direct incorporation of Tc into glasses is impractical because it is volatilized in the vitrification process. The present inventors took advantage of the long-term stability of certain types of polyoxometalate complexes and their reduced derivatives, the heteropoly"blues"and "browns". Heteropolyanions can be thought of as synthetic models for minerals, and control over stoichiometry and solubility is well-known. In particular, the present inventors previously elected to incorporate 99Tc. into these anions following established chemistry for rhenium. These anions are easily prepared in solution, and are easily reduced to heteropoly blues. It was theorized that reduced heteropolyanions derived materials wouldbe attractive-waste'foriris for-Tc since they-potentially would be stabilized for long term storage by the reducing environment provided by technetium's radioactive decay to stable ruthenium.

Technetium and rhenium are sufficiently similar in atomic/ionic radii and redox activity to allow Re to be potentially considered as a non-radioactive surrogate for Tc in polyoxometalate chemistry. For example, the ionic radii of Tc4+ and Re4+ are 0. 77 and 0. 785 A respectively, and the reduction potentials, as far as they are known, show few differences. The pertechnetate anion, Tc04-, is a slightly stronger oxidant than Re04 (E#, MO4-/MO2, +0. 74 vs. 0. 51 V) but both are much weaker than permanganate. In oxidic environments Re and Tc show many similarities to Mo and W : the dioxides of all four elements are essentially isostructural, as are W03 and ReO3.

Also, the present inventors previously investigated the possibility that polyoxometalates incorporating Re instead of Mo or W could be synthesized. The rationale for this research was the hope that neutral polyoxometals would result by replacing enough WVI atoms by ReVII. Several substituted Keggin anions, containing ReVO3+ moieties in place of wVIo4+, were synthesized, [XW11ReO40]n- (X=P, Si, B), and [SiWl0Re2040] 6~. These stable complexes could easily be oxidized to isostructural Re and Re analogues, and reduced to ReIV and ReIII species. See Ortega et al, Inorg Chem.

23 : 3292 (1984). More recently, Abrams et al (Inorg. Chim. Acta. 180 : 9 (1991)) have reported analogous Tc derivatives, [PW11TcO40] [SiWnTc04o]',and[PWnTcNC].

Oxidation of the first of these complexes to the TcVI derivative is reported to occur at +0. 84 V (vs. Ag/AgCl ; acetonitrile solution) ; the corresponding potential for the Re complex is +0. 13 V.

A specific example of a previously reported method for synthesizing polyoxometalates containing rhenium comprises complexes analogous to those described above in which ReO3+ has replaced one or more WO4+ or MoO4+ groups. It is striking that the 6-coordinate ionic radii of Re5+ and W6+ are identical (0. 74 A). The general procedure for such syntheses is the reaction of appropriate lacunary anions or other precursor species with Re04 and a reducing agent such as I-or Sn. Such a procedure (M04-plus reductant) is routinely used for the synthesis of complexes with Tc03+ cores for radiopharmaceutical applications. An alternative convenient source of Re (Tc) is Mol62-. In addition to the lacunary anions that were used in previous work, several other heteropoly species have been investigated. These include more highly-charged monovacant lacunary anions such as [PzWiyOi]'; divacant species γ-[SiW10O36]8 (Canny et al, Inorg. Chem. 25 : 2114 (1991) ; Teze et al, Inorg. Synth. 27 : 85 (1990) ; Canny et al, Inorg Chem. 30 : 976 (1991) ; Zhang et al, Inorg. Chem. 35 : 30 (1996)), and anion 10 (which is both a cryptand and a lacunary anion with binding sites for two metal cations (or oxocations) in addition to the encrypted central atom) ; and trivacant species such as A- [SiW9034] 1°- (Herve et al, Inorg Chem. 16 : 2115 (1977)) ; B-[XIIIW9O33]9- (X=As, Sb) (Tourne et al, Hebd. Seances Acad. Sci., Ser. C, 277 : 643 (1973) and Jeannin et al, J Am.

Chem. Soc. 1-03 : 1664- (1981)) ; and WisC' (Contant et al, J. Inorg. Nucl. Chem.

43 : 1525 (1981) and Contant, Inorg. Synth. 27 : 108 (1990)). Synthesis of the starting lacunary anions is a straightforward procedure to one of skilled in the art.

A second previously reported method for incorporation of Re (Tc) in polyoxometalates is to target heteropoly derivatives of Re starting with simple tungstate or molybdate anions and Re species. For example, salts of [ReCl6] 2~ are isostructural with those of [PtCl6] 2-. Numerous examples of"Anderson"-type heteropolyanions of PtIV, [PtM6024] 3- (M=Mo, W) have been prepared (Lee et al, Acta Crystallogr, Sec. C 40 : 5 (1984) ; Lee et al, Chem. Lett. 1297 (1984)).

The present inventors have recently been investigating polyoxotungstate complexes with lanthanides and actinides in connection with possible applications in the sequestration and immobilization of these species in nuclear wastes. Although polytungstates are thermally stable and radiation-resistant, their instability in highly alkaline solutions renders them inappropriate for certain waste streams, e. g. alkaline tank wastes. In contrast polyoxoanions of niobium and tantalum are stable only in basic media (pH>10).

What is needed in the art are novel polyoxometalates and methods of immobilizing technetium which overcome the problems associated with existing methods. Namely, a method is required that results in the production of a form of technetium that is stable and which is easily recovered from a material such as nuclear waste, facilitating its separation or removal therefrom. The present invention provides such polyoxometalates and process.

SUMMARY OF THE INVENTION An object of the present invention is to provide novel tricarbonyl metal derivatives of hexametalate anion or salts thereof which serve as surrogates for technetium radioactive cations. The derivatives are of the formula [M6019IM' (CO) I} nl ('-n)-, wherein M is Nb or Ta, M'is Mn or Re and n is 1, 2, 3 or 4. Preferred tricarbonyl metal derivatives include tricarbonyl derivative is selected from the group consisting of : <BR> <BR> [Re(CO)3Nb60i9f';[ReCTaeCf';[Mn(CO)3Ta<Oi9]'';[MCCNbcCtra ns-<BR> <BR> 1WO3J26196C1S-ylly3126196 trans-[{Mn(CO)3}2Ta6O19]6-; cis-[{Mn(CO)3}2Ta6O19]6-; cis-[{Re(CO)3}2Nb6O19]6-; trans-[{Re(CO)3}2Nb6O19]6-; trans-[{Re(CO)3}2Ta6O19]6-; cis-[{Re(CO)3}2Ta6O19]6-.

Another object of the invention is to provide a method for immobilizing technetium (Tc) which comprises binding Tc cations to a nucleophilic polyoxometalate anion, wherein Tc is stabilized by pi-acceptor through ion exchange of Re or Mn, initially present in the polyoxometalate anion.

Another object of the present invention is to provide a method for immobilizing technetium (Tc) which comprises binding Tc cations, stabilized by pi=acceptor ligands, to highly nucleophilic polyoxometalate anions.

A more specific object of the invention is to provide Tc compounds in the form of monovalent Tc cations that bind to nucleophilic polyoxometalate anions to form stable, insoluble, non-volatile products. Another specific object of the invention is to provide a novel method for removal of technetium (Tc) from industrial compositions, in particular nuclear waste compositions, by binding positively charged (Tc (I) cations, stabilized with appropriate pi-acceptor ligands (e. g., CO and isonitriles) to nucleophilic polyoxymetalate anions and removing the resultant insoluble form of technetium, which is immobilized to a polyoxyanion derivative.

Still another object of the invention is to provide novel storage stable forms of technetium, i. e., insoluble salts comprising technetium and polyoxoanions or polyoxyanion polymers.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1. Oxygen-17 NMR spectra of (a) MnTa6, (b) MnNb6, (c) ReTa6, and (d) ReNb6 Figure 2. Oxygen-17 NMR spectra of trans isomers of (a) Re2Nb6, (b) Mn2Nb6, and (c) Re2Ta6 Figure 3. Oxygen-17 NMR spectra of cis isomers of (a) Re2Nb6, (b) Mn2Nb6, and (c) Mn2Ta6 Figure 4. The structure of trans-[Nb6019 {Re (CO) 3} 2] 6~ in (a) polyhedral, and (b) thermal-ellipsoid (50 %) representations.

Figure 5. The structure of cis-[Nb60l9tRe (CO) 3} 2] 6- in (a) polyhedral, and (b) thermal-ellipsoid (50 %) representations.

Figure 6. Infrared spectra of Nb6, ReNb6, t-Re2Nb6, and c-Re2Nb6 Figure 7. TGA curves of t-Mn2Ta6 (Mn) and t-Re2Nb6 (Re) DESCRIPTION OF THE PREFERRED EMBODIMENTS The term"anion"or"anionic"refers to any negatively charged ion, for example, Mn04-or Tc04-.

"Cation"refers to any positively charged ion.

The term"Curie (Ci)"refers to the unit of radioactivity equal to 3. 7 x 101° disintegrations per second or 3. 7 x 101° becquerel (Bq). A common unit used in environmental measurements is the picocurie (pCi) which is equal to l/10 l2 Ci or 0. 037 disintegrations per second or 0. 037 Bq.

The term"half-life"is used to mean the time it takes for half of a radioactive material to radiate energetic particles and rays and transform to new materials. For example, cesium (Csl37) has a half life of thirty years. After this time, half of the Cl 137 decays to a non-radioactive stable nuclide, barium (Ba-137). The term"hazardous waste" refers to any solid wastes that pose substantial or potential threats to public health or the environment including any waste that meets the criteria specified in 40 CFR 260 and 261 : (a) is specifically listed as a hazardous waste by EPA ; (b) exhibits one or more of the characteristics of hazardous waste (ignitability, corrosivity, reactivity, and/or toxicity) ; (c) is generated by the treatment of hazardous waste ; or is contained in a hazardous waste.

"High Level Radioactive Waste (HLW)"refers to radioactive waste material that results from the reprocessing of spent nuclear fuel, including liquid waste produced directly from reprocessing and any solid waste derived from the liquid that contains a combination of transuranic and fission product nuclides in quantities that require permanent isolation. HLW is also a mixed waste because it has highly corrosive components or has organics or heavy metals that are regulated under RCRA. HLW may include other highly radioactive material that NRC, consistent with existing law, determines by rule requires permanent isolation The term"solid waste"refers to any solid, semi-solid, liquid, or contained gaseous materials discarded from industrial, commercial, mining, or agricultural operations, and from community activities. Solid waste includes garbage, construction debris, commercial refuse, sludge from water supply or waste treatment plants, or air pollution control facilities, and other discarded materials. Solid waste does not include solid or dissolved materials in irrigation return flows or industrial discharges which are point sources subject to permits under section 402 of the Clean Water Act or source, special nuclear, or byproduct material defined by the AEA.

The term"vitrification"refers to the process of converting materials into a glass- like substance, typically through a thermal process. Radionuclides and other inorganic are chemically bonded in the glass matrix. Consequently vitrified materials generally perform very well in leach tests. EPA has specified, under the land disposal restrictions, vitrification to be the treatment technology for high-level waste.

As discussed previously, initial investigations were carried out with rhenium as a Tc-surrogate to ascertain whether such methods would result in rhenium polyoxyanion compounds having desirable properties providing for their separation. For example, Tungstorhenates incorporating Re (V) and Re (IV) have been characterized. However, polyniobates and niobotungstates offer the advantages of stability in alkaline solutions which constitute the majority of tank wastes. The first example of a nioborhenate (IV) complex, [Re (Nb6OI9) 2] 12 has been isolated from alkaline solution but has proved difficult to crystallize for structural confirmation. Furthermore, Re is slowly oxidized and released from this material. A much more promising strategy has recently been discovered by the present inventors. Specifically, the discovery of the present invention is that positively charged Tc (I) cations can be stabilized by appropriate pi-acceptor ligands such as CO or isonitriles (RNC) which stably bind to nucleophilic polyoxometalate anions. Moreover, and advantageously, it has been found that the resultant complexes are highly stable in different thermal, hydrolytic and oxidizable environments. Also, it has been discovered that these reactions can be effected in aqueous solutions under hydrothermal conditions, e. g., about 130°C in a closed container.

More specifically, it has been found that alkaline (pH-l2) aqueous solutions of NbgO'react with Re (I) carbonyl complexes under hydrothermal conditions to form [Nb6OIRe (CO) 3 ; f' and [Nb60i9 {Re (CO) 3} 6- (syn and anti isomers). These novel complexes have been characterized by 17 NMR spectroscopy and single crystal X-ray structural analysis. The solid salts are thermally stable to 400°C (loss of CO, not Re) and are inert to oxidation.

According to the invention, a typical reaction involving Tc is : [Nb60t9'+ 2 Tc (CO) 5Br--> [NbsOTcCf'(1)+2Br'+4CO The invention is applicable using different technetium starting materials. For example, other technetium compounds which can be used as starting materials (synthons) for such reactions include, e. g., [Tc (CO) 3 (H20) 3, [Tc (NCR) 6] +, and (C5Me5) Tc (CO) 3.

Methods for synthesis of such compounds are well knownFor example, simple, efficient methods of synthesis of the latter compounds from Tc04-have been developed in the context of nuclear medicine (use of 99mTc, an isotope with a 6-hour half-life, for organ imaging). The formation of a compound such as (1) above is favored by the electrostatic attraction between the Tc (I) cation (e. g. [Tc (CO) 3 (H2O) 3] +) and the negatively charged oxygens on the surface of the polyoxoanion (e. g. [Nb6OI9] 8-). However, the final product (1) contains Tc covalently attached to the polyanion through three strong Tc-O bonds and is extremely stable.

Other polyoxometalate anions containing other early transition elements such as Nb (V), Ta (V), Mo (VI), W (VI), W (IV), Ti (IV) and Zr (IV), or combinations of these elements can be used. The resulting compounds may be discrete polyoxoanion derivatives (such as (I)) which may be isolated and stored as insoluble salts, or the starting polyoxoanion itself may be an insoluble polymeric material (an example is [KMo5016H (H, (HO) o). In such cases, the Tc atoms will become covalently attached to the polymer and remain permanently immobilized.

Given that the subject invention provides stable, insoluble technetium compounds, the invention is particularly well suited for removal of technetium ions, e. g., nuclear waste materials. Essentially, such methods will comprise treating said waste under the above-described alkaline conditions to provide insoluble technetium salts which are then recovered and stored. As noted, these salts are stable at high temperature (stable up to 400°C) and inert to oxidation. Thus, the present invention provides complexes which should be excellent materials for separation and storage of technetium from nuclear waste. Also, the fact that they are stable under alkaline conditions is a particular advantage as this corresponds to conditions of the majority of nuclear tank washes.

The new niobate and tantalate derivatives reported here are easily prepared in good yield in aqueous solution starting with [M (CO) 3 (CH3CN) 3] + or under hydrothermal conditions with [M (CO) 5Br]. The compositions and structures of the new complexes are established by elemental analysis, infrared and oxygen-17 NMR spectroscopy, and by single crystal X-ray diffraction of five salts.

Oxygen-17 NMR spectroscopy proved to be the simplest way to confirm the structures of the product anions, see Figures 1-3, and Table 1. The salts were soluble enough to allow accumulation of spectra of unenriched samples within 20 h. All symmetry-distinct oxygen atoms could be detected and assignments are shown in the Figures. All assignments were made on the basis of peak intensities and proximity of peaks to corresponding peaks of Nb6 and Ta6. Some spectra revealed the presence of ClO4-(# # 290 ppm) as an impurity, and spectra of enriched samples of ReNb6 showed signals from traces of c-and t-Re2Nb6. The formation of 2 : 1 complexes and the existence of both cis and trans isomers was first deduced from the NMR spectra.

Table 1. Oxygen-17 NMR Chemical Shifts Compound Terminal Bridging Central Carbonyl Nb6 600 395 26 Ta6 483 331-34- MnNb6 615, 633 128, 410, 428 35 378 ReNb6 629, 645 156, 408, 435 40 333 MnTa6497, 516. 91, 346, 357..-2L-380- ReTa6 502, 521 121, 349, 361 -17 334 c-Mn2Nb6 ? A, 659, 697 145, ?, 447, 454 47 382, 383 c-Re2Nb6 659, 190, 719 465 54 334, 190, c-Mn2Ta6 ?, 541, ? 103, ?, 367, 371 ? 384, 386 t-Mn2Nb6 663 143, 447 46 382 t-Re2Nb6 679 169, 458 53 335 t-Re2Ta6 553 133, 379 336 Signal not reliably observed X-ray analysis of single crystals of potassium salts of trans isomers of Re2Nb6, Mn2Nb6, and Re2Ta6 and of cis isomers of Mn2Nb6 and Re2Nb6 are summarized in Tables 2 and 3. The structures of trans-and cis-isomers are shown in polyhedral and thermal-ellipsoid representations in Figures 4 and 5. The metrical data listed in Table 3 are for the most part unexceptional. Attachment of [M (CO) 3] + to the anions results in a "closing up"of the three surface oxygen atoms involved. The non-bonded O---O contacts are-0. 2 A shorter than those in corresponding 03 sets that are not bonded to M (2. 777- 2. 851). This presumably reflects the reduction in charge density on the bonded oxygen atoms. At the same time the O-M-O angles are reduced from an idealized 90° to 75-80°, a distortion dictated by the lengths of the M-O bonds.

Table 3. Selected average bond-lengths (A) and-angles (°). M = Nb, Ta ; M'= Mn, Re t-Re2Nb6 t-t-Re2Ta6 c-Re2Nb6 c-Mn2Nb6 Mn2Nb6 M-1. 765 (5) 1. 776 (5) 1. 79 (2) 1. 765 (6) 1. 771 (5) O (terminal) M-O (M) 1. 964 (4) 1. 971 (4) 1. 96 (1) 1. 920 (6)-1. 931 (1)- 2. 010 (6) 2. 004 (4) M-O (M, M') 2. 048 (4) 2. 044 (4) 2. 06 (1) 2. 017 (6)- 2. 003 (4)- 2. 092 (6) 2. 066 (4) M-O (M5) I 2. 3972 (6) 2. 3967 (6) 2. 397 (1) 2.397 (6) I 2. 386 (4) M'-C 1. 894 (7) 1. 804 (8) 1. 88 (3) 1. 904 (9) 1. 793 (8) M'-O(M2) 2.176(5) 2.085(5) 2.17(2) 2.178(6) 2.070(5) CM'C 88. 6 (4) 88. 8 (4) 90 (1) 89. 4 (4) 88. 9 (3) OM'O 74. 8 (2) 79. 2 (2) 75. 6 (7) 74. 9 (2) 79. 0 (2) The cis isomers of the 2 : 1 complexes have two open 03 sets that could be used for -attachment of additional M (GO) 3--groups. All four such positions are occupied in the neutral species [{C5Me5)Rh}4V6O19]. Attempts to introduce more than two M (CO) 3 groups to the niobate and tantalate systems have so far been unsuccessful.

Representative infrared spectra are shown in Figure 6 and these provide the quickest way to identify the three structural types. Vibrational frequencies for all the new complexes are listed in the Supporting Information, The simple metal-oxygen stretching pattern (400-900 cm~l) of the original hexametalate anionsl6 is split into more components by the lower symmetry of the M (CO) 3 derivatives, and a new weak band at ~460 cm~l is assigned to the Mn (Re)-C vibrations. lob In addition two sets of CO stretching bands are seen at ca 1900 and 2000 cm~1. Similar bands are observed for a number of fac- [M (CO) 3L3] +, M=Mi, Re complexes.

Unlike the underivatized hexametalates, which are stable only in strongly basic media (precipitation of the oxides begins below pH-10, solutions of the Mn and Re derivatives show no sign of precipitation when the pH is lowered to 4. Cyclic voltammograms of all the compounds in 0. 1 M sodium acetate show no redox features except an irreversible oxidation process at +1. 5 V (Re) and+0. 9 V (Mn). Similar features are observed in voltammograms of the corresponding [M (CO) 3 (CH3CN) 3] + cations.

Representative TGA curves for t-Mn2Nb6 and t-Re2Nb6 are shown in Figure 7.

The other complexes behave similarly. The initial weight loss corresponds to the loss of water of crystallization. A second endothermic process starts at 400-4500C for the Re compounds and-at-200°C for the Mn-compounds, and correspondstotheloss of the CO ligands. A third weight loss (Re compounds only) above 700°C is assumed to be associated with the loss of Re207 formed by oxidation or disproportionation.

EXAMPLES Syntheses.

Potassium hexaniobate, K7HNb6O1913H2O (Nb6) and potassium hexatantalate, K8Ta6Ol917H2O (Ta6) were prepared by literature methods and characterized by infrared spectroscopy and l7O-NMR spectroscopy. Nb6 IR, cm'' : 856 (vs), 777 (s) ; 669 (s) ; 528 (s) ; 418 (vs) ; 170-NMR, ppm : 26 (O (M6) ; A) ; 395 (O (M2) ; B) ; 600 (O (M) ; C) ; Ta6 IR, cm l : 854 (s), 837 (s), 700 (s), 536 (s) ; 17O-NMR, ppm :-34 (O (M6) ; A) ; 331 (O (M2) ; B) ; 483 (O (M) ; C). Sodium salt of Nb6 was obtained from a solution of the potassium salt by precipitation with 5 M NaCl. Dirhenium decacarbonyl, Re2 (CO) IO, and pentacarbonylbromomanganese, Mn (CO) 5Br, were purchased from Aldrich and used without further purification. Pentacarbonylbromorhenium, Re (CO) sBr, triscarbonyl- trisacetonitrilerhenium perchlorate, [Re (CO) 3 (CH3CN) 3] C104 and triscarbonyltrisacetonitrilemanganese perchlorate, [Mn (CO) 3 (CH3CN) 3] C104 were prepared by literature methods and characterized by infrared spectroscopy and 170 NMR spectroscopy. Re (CO) 5Br IR, cm~l : 2151 (m) ; 2059 (vs), 2035 (vs) ; 1974 (vs), 1964 (vs) ; 1122 (m) ; 1109 (w) ; 1004 (vw) ; 939 (vu) ; 912 (vw) ; 588 (vs) ; 553 (m) ; l7O-NMR (CD3CN), ppm : 357 (sh), 356 ; 338 (CO); [Re(CO)3(CH3CN)3]ClO4 IR,cm-1: 2364 (w), 2327 (w), 2298 (w), 2045 (vs), 1934 (vs), 1419 (w), 1369 (w), 1143 (sh), 1091 (vs), 1035 (w), 910 (w), 648 (sh), 625 (s), 588 (m), 540 (w), 482 (w) ; 170_NMR (CD3CN), ppm : 342 (CO), [Mn (CO) 3 (CH3CN) 3] ClO4 IR, cm'' : 2364 (w), 2327 (w), 2300 (w), 2057 (vs), 1959 (vs), 1423 (w), 1373 (w), 1093 (vs), 1037 (m), 681 (m), 625 (s), 528 (m), 457 (m) ; 170 NMR (CD3CN), ppm : 389 (CO).

Example I K7 [Re (CO) 3Nb6Ol9] (ReNb6). Deionized water was used throughout all syntheses.

One mmol (0. 4067 g) Re (CO) 5Br was added to a solution of 1. 37 g (lmmol) Nb6 in 2 mL water contained in a teflon-lined Parr acid digestion bomb, the bomb was sealed and the mixture was heated to 130°C for 17 hours. Upon cooling the resulting mixture, about 10 mg of insoluble unreacted Re (CO) 5Br was filtered off and addition of 15 mL ethanol to the filtrate (Vadjusted- mL) yielded 1. 19 g (71 %) of crude product. The impurity, about 10 % of trans-K6 [{Re (CO) 3} 2Nb6Ol9] (t-Re2Nb6) was removed by dissolving the solid in 2. 5 mL of water and adding 3 mL 5M NaCl to precipitate pure ReNb6. Anal. Obs. (Calc. for Na7 [Re (CO) 3Nb6Ol9]. 9H20) : Na, 12. 1 (11. 06) ; Re 12. 7 (12. 8) ; Nb 37. 3 (38. 3) C 2. 52 (2. 47).

Example II K7 [Re (CO) 3Ta60i9] (ReTa6). One-half mmol (0. 1809 g) [Re (CO) 3 (CH3CN) 3] C104 was dissolved into a solution of 1. 0038 g (0. 5 mmol) Ta6 in 10 mL of water, which had previously been deaerated with a stream of N2 for 20 min. The reaction mixture, in a 100-mL single-necked flask fitted with a reflux condenser, was heated to 60°C under a positive pressure of N2 for 2. 5 h. Upon cooling 60 mg of white insoluble material was filtered off, and addition of 15 mL ethanol to the filtrate (Vadjusted==15 mL) yielded 0. 70 g (64 %) product. Anal. Obs. (calc. for K7 [Re (CO) 3Ta6O19]. 9H20) : K 12. 7 (13. 1) ; Re 9. 10 (8. 90) ; Ta 50. 2 (51. 8) ; C 2. 10 (1. 72).

Example III K7 [Mn (CO) 3Ta601ol (MnTa6). The procedure for ReTa6 was repeated using [Mn (CO) 3 (CH3CN) 3] C104. After heating, 20 mg insoluble material was filtered off and the yellow product (0. 57 g, 55%) was recovered from the filtrate (Vadjusted-lS mL) by precipitation with 15 mL ethanol. Anal. Obs. (calc. for K7 [Mn (CO) 3Ta6Ol9]. 15H2O) : K 13. 22 (13. 21) ; Mn 2. 67 (2. 65) ; C 1. 76 (1. 74) ; Ta 52. 30 (52. 39).

Example IV K7 [Mn (CO) 3Nb6019] (MnNb6). The procedure for ReTa6 was repeated using [Mn (CO) 3 (CH3CN) 3] ClO4 and Na7HNb601915H2O. After heating the reaction mixture (V=40 mL) at 90°C for 3 h., a small amount of insoluble material was filtered off, and the volume of the filtrate was-reduced to 15 mL on a rotary evaporator. The'yellow product (1. 00 g, 70 %) was obtained by precipitation with 15 mL ethanol. Anal. Obs. (calc. for K7 [Mn (CO) 3Nb60i9]. 12H20) : K 17. 51 (17. 79) ; Mn 3. 25 (3. 57) ; C 2. 30 (2. 34) ; Nb 37. 24 (36. 24).

Example V trans/cis-K6l {Mn (CO) 3} 2Nb6019] (t/c-Mn2Nb6). The procedure for ReTa6 was repeated using two mmol (0. 72 g) of [Mn (CO) 3 (CH3CN) 3] C104 and one mmol (1. 37 g) of K7HNb6Ol913H2O. After stirring the reaction mixture (V=40 mL) at room temperature for 4. 5 h, a small amount of insoluble material was filtered off. The volume of the filtrate was reduced to 20 mL using a rotary evaporator, and 20 mL ethanol was added to the filtrate to produce a yellow precipitate (0. 48 g, 29 %, 75 % t-Mn2Nb6, 25 % c-Mn2Nb6). The resulting filtrate, still yellow, was evaporated to 5 mL, and pale yellow material (KC104) was filtered off. c-Mn2Nb6 (0. 43 g, 25 %) was precipitated with 7 mL ethanol.

The relative amounts of trans vs. cis isomers are influenced by synthesis temperature. When previously described procedure was repeated by refluxing the reaction mixture at 80°C for 3 h, first precipitation with ethanol afforded 0. 89 g of yellow product (51 % ; 25 % t-Mn2Nb6, 75 % c-Mn2Nb6).

Crystals suitable for x-ray study were grown by a combination of slow cooling and slow ethanol vapor diffusion into an aqueous solution. Four-hundred~èighty milligrams of t-Mn2Nb6 was dissolved in 2. 5 mL of water. Two types of crystals appeared after 2-3 weeks of storage in a refrigerator at 7 0 C. Small yellow crystals of t-Mn2Nb6 (0. 17 g) formed. In addition, 0. 06 g of large yellow needle type crystals of c-Mn2Nb6 was recovered. Anal. Obs. (calc. for K6 [{Mn (CO) 3}2Nb6O19].8H2O): K 15. 60 (15. 46) ; Mn 7. 20 (7. 24) ; C 5. 10 (4. 74) ; Nb 36. 2 (36. 7). Four hundred thirty milligrams of c-Mn2Nb6 was dissolved in 2. 5 mL of water. Large needle type yellow crystals of c-Mn2Nb6 (0. 16 g) formed after 2-3 weeks of storage in a refrigerator at 7 0 C. Anal. Obs. (calc. for K6[{Mn(CO)3}2Nb6O19].23H2O) : K 13. 30 (13. 12) ; Mn 6. 25 (6. 15) ; C 4. 15 (4. 03) ; Nb 29. 9 (31. 2).

Example VII cis-K6 [{Mn (CO) 3} 2Ta6019] (c-Mn2Ta6). The procedure for ReTa6 was repeated using 0. 56 mmol (0. 2006 g) of [Mn (CO) 3 (CH3CN) 3] C104 and 0. 56 mmol (1. 15 g) of MnTa6. Several drops of 3M KOH were added to adjust the pH of water to 11 before dissolving MnTa6. After refluxing the reaction mixture (V=10 mL) at 100°C for 2 h, about 60 mg of insoluble material was filtered off, and the yellow product (0. 60 g, 49 %) was recovered from the filtrate (Vadjusted= 10 mL) by precipitation with 15 mL ethanol.

Anal. Obs. (calc. for K6 [{Mn (CO) 3} 2Ta60l9] 12 16H2O) : K, 10. 52 (10. 71) ; Mn, 5. 21 (5. 02) ; C, 3. 22 (3. 29) ; Ta, 48. 4 (49. 6).

Example VIII trans/cis-K6 [{Re (CO) 3} 2Nb6019] (t/c-Re2Nb6). The procedure for ReNb6 was repeated using one mmol (1. 37 g) of Nb6 and two mmol (0. 82 g) of Re (CO) 5Br. After cooling the resulting mixture 0. 84 g (44 %) of a white precipitate (mostly t-Re2Nb6 with impurity of unreacted Re (CO) sBr) was recovered. t-Re2Nb6 was purified by dissolving in 2 mL of water, filtering off 10 mg of unreacted Re (CO) 5Br, and precipitating t-Re2Nb6 with ethanol. To the filtrate left from hydrothermal synthesis 5 mL of ethanol was added producing 0. 49 g (25 %) of white precipitate (-75 % c-Re2Nb6,-25 % t-Re2Nb6).

Crystals suitable for x-ray study were obtained similarly to the manganese compounds. Eight hundred ten milligrams of t-Re2Nb6 was dissolved in 5 mL of water.

Seven hundred twenty milligrams of colorless block crystals of t-Re2Nb6 were recovered after storage in a refrigerator for 8 weeks at 7°C. Anal. Obs. (calc. for K6 fRe (CO) 3} 2Nb6Ol9]. 15H20) : K 12. 42 (12. 31) ; Re 19. 55 (19. 53) ; C 3. 81 (3. 78) ; Nb 29. 23 (29. 24). Six hundred twenty milligrams of c-ReiNb6 was dissolved in 3 mL of water. Three hundred forty milligrams of large colorless needle crystals of c-Re2Nb6 was recovered after refrigerating for 6-8 weeks at 7 DC. Anal. Obs. (calc. for K6 [f Re (CO) 3}2Nb6O19].19H2O) : K 11. 83 (11. 86) ; Re 1'8. 72 (18. 82) ; C 3. 62 (3. 64) ; Nb 28. 11 (28. 18).

An alternative procedure for trans-K6 [{Re (CO) 3} 2Nb6019l (t-Re2Nb6). The procedure for ReTa6 was repeated using one mmol (1. g) of [Re (CO) 3 (CH3CN) 3] C104. After heating the reaction mixture (V=40 mL) at 60 0 C for 1. 5 h., a small amount of insoluble material was filtered off. The volume of the filtrate was reduced to 20 mL using a rotary evaporator, and 0. 95 g (50 %) of white t-Re2Nb6 was recovered after precipitation with 20 mL ethanol.

Example XI trans-K4Na2 [{Re (CO) 3} 2Ta60l9] (t-Re2Ta6). The procedure for c-Mn2Ta6 was repeated using 0. 32 mmol (0. 71 g) of potassium salt of ReTa6 and 0. 32 mmol (0. 1591 g) of [Re (CO) 3 (CH3CN) 3] C104. Several drops of 3 M NaOH were added to adjust pH of water >10 before dissolving ReTa6. After refluxing the reaction mixture (V=12 mL) at 100 for 1 h, 0. 53 g (68 %) of white t-Re2Ta6 was recovered by precipitation with 12 mL ethanol. Single crystals were grown using the same procedure as for t-Mn2Nb6 and t- Re2Nb6. Anal. Obs. (calc. for K4Na2 [{Re (CO) 3} 2Ta6Ol9] 12 l lH2O) : K, 7. 31 (6. 71) ; Na, 1. 98 (1. 97) ; Re, 15. 7 (16. 0) ; C, 3. 23 (3. 09) ; Ta, 45. 32 (46. 58). 17 enrichment.

Example X The procedure for synthesis of ReNb6, t/c-Re2Nb6 was followed using instead of deionized water a mixture of 1. 5 mL of 99% D20 and 1. 0 g of 10 % enriched water for ReNb6 enrichment and a mixture of 2.5 mL of 99 % D20 and 0. 53 g of 10 % enriched water for t/c-Re2Nb6 enrichment.

Instrumental.

An approximate sphere of data was collected on a Siemens SMART 1K CCD system. Crystal stability was monitored by recollection of the first fifty frames after data collection was finished. No significant decay was observed. Crystallographic data are in Table 2. The structures were solved by direct methods using SHELXTL. Hydrogen at were not included in the model.

Table 2. Crystal data. t-Re2Ta6, 1 t-Re2Nb6, 2 tMn2Nb6, 3 c-Mn2Nb6, 4 c-Re2Nb6, 5 Empirical K4 Na2 Re2 C6 Ta6 K6 Re2 C6 Nb6 O38 K6 Mn2 C6 Nb6 K6 Mn2 C6 Nb6 O50 K6 Re2 C6 Nb6 O54 Formula weight 2312.70 1870.76 1590.19 1824.40 2158.98 Space group C2/m (No.) C2/m (No. 12) C2/m (No. 12) P1 (No. 2) $P21/c (No. 14) Unit Cell, Å, ° a= 17.648(3) a= 17.724(1) a=17.812(2) a=10.2617(6) a=21.867(2) b= 10.0561(14) b=10.0664(6) b=10.098(1) b=13.4198(8) b=10.3085(9) c= 13.1714(19) c=13.1965(7) c=13.109(2) c=21.411(1) c=26.780(2) ß= 112.531(2) ß=112.067(1) ß=112.733(2) α=72.738(1) ß=108.787(1) ß=85.591(1) γ=83.501(1) Volume, Å3 2159.1(5) 2182.0(2) 2174.7(5) 2794.7(3) 5714.8(9) Z 2 2 2 2 4 T, °C -102 -102 -102 -102 -102 #, Å 0.71073 0.71073 0.71073 0.71073 0.71073 Calc. Density, 3.557 2.853 2.428 2.168 2.509 M 21.217 7.718 2.760 2.179 5.931 Ra 0.0588 0.0307 0.0484 0.0583 0.0574 wR2b 0.1764 0.0826 0.1338 0.1927 0.1676 17 NMR spectra were collected on Bruker AM 300 spectrometer. The offset frequency was 40. 687 MHZ, bandwidth was 50 KHz, and repetition rate was 10 Hz.

All spectra were collected in deuterated water, except for the spectra of Re (CO) 5Br, Re (CO) 3 (CH3CN) 3ClO4, Mn (CO) 3 (CH3CN) 3C104 which were collected in deuterated acetonitrile. Collection time for unenriched samples was about 20 h.

IR spectra were obtained on Nicolet 7000 spectrometer with resolution of 2 cm' . All samples were prepared as KBr pellets.

TGA curves were obtained on TGA 2050 analyzer made by TA Instruments from room temperature to 950°C under nitrogen atmosphere. Alumina pans were used for all measurements.

DSC curves were obtained on DSC 2910 calorimeter made by TA Instruments from room temperature to 725°C under nitrogen atmosphere. Platinum pans were used for all measurements.

Cyclic voltammetry curves were obtained for all compounds in aqueous solution. 0. 1 g of the corresponding compound was dissolved in 10 mL of deaerated water forming-0. 02 M solution. 0. 1 M Na (CH3COO) was used as a supporting electrolyte. Platinum wire was used as a counting electrode, glassy carbon was used as a working electrode, and Ag/AgCl saturated electrode was used as a reference electrode.

Chemical analyses were performed by Kanti Technologies Inc., 43 Old Falls Blvd., N. Tonawanda, NY 14120.

Example dz Selective sequestration of Ln3+/An4+ by [NaP5W30O110]14- The sodium cation that occupies the central cavity of this doughnut-shaped polyoxotungstate can be replaced only by cations of the same size, i. e. Ln3+, An4+ (U- Cm), Ca2+. Other cations are excluded. In aqueous solution (neutral or mildly acidic) at 160°C, Nd3+ for example is incorporated in the presence of the major components of tank wastes, i. e. a 200-fold molar excess of Na+, and in the presence of Fe3+ and Al3+.

The encrypted metal cations are released by hydrothermal treatment under strongly acidic conditions.

Example XII Polytungstate complexation of U0 ? 2+ Uranyl (and other actinyl) cations are stable in aqueous solution under normal atmospheric conditions. The first examples of polytungstate complexes of UO22+ have been synthesized and structurally characterized. Examples include [Na2(U02)2(PW9034)2f'and[(UCCSsWBoOiosf'.Anion [Na2 (UO2) 2 (PW9034) 2] 12-iS formed in solutions of high [Na+] and is extractable into organic solvents.

Example XIII Conversion of tungstolanthanide and-actinide salts into tungsten bronzes Under relatively mild conditions (N2 or H2 atmosphere, 500-700°C) ammonium salts of anions such as [((HCeAssWsoO]'are cleanly converted into the cubic bronzes (Ln/An) xW03. This method of synthesis represents a major improvement over conventional methods (ground mixtures of oxides and tungsten powder, 1000°C). The chemical stability of the bronzes as candidates for potential waste forms is currently under investigation.

Exemple mu Technetium recoverv and storage Initial investigations were carried out with rhenium as a Tc-surrogate.

Tungstorhenates incorporating Re (V) Re (IV) have been characterized but polyniobates and niobotungstates offer the advantages of stability in alkaline solutions which constitute the majority of tank wastes. The example of a nioborhenate (IV) complex, [Re (Nb60i9)2]'has been isolated from alkaline solution but has proved difficult to crystallize for structural confirmation. Furthermore, Re is slowly oxidized and released from this material. A much more promising strategy has recently been discovered. Alkaline (pH-12) aqueous solutions of Nb60198-react with Re (I) carbonyl complexes under hydrothermal conditions to form [Nb6OI9Re (CO) 3] 7- (V) and [Nb6019 {Re (CO3} 2] 6~ (syn and anti isomers, VI). The new complexes have been characterized by 17 NMR spectroscopy and single crystal X-ray structural analysis.

The solid salts are thermally stable to 400°C (loss of CO, not Re) and are inert to oxidation. Known synthetic routes to Ml precursor species such as M (CO) 3+ from Re (Rc) O4~, and the variety of nucleophilic polyoxometalate anions available, suggest that complexes such as V and VI will prove to be excellent vehicles for separation and storage of Tc wastes.

While the invention has been described in terms of preferred embodiments, the skilled artisan will appreciate that various modifications, substitutions, omissions and changes may be made without departing from the spirit thereof. Accordingly, it is intended that the scope of the present invention be limited solely by the scope of the following claims, including equivalents thereof.