BACKGROUND OF THE INVENTION It is known to produce tetrakis (pentafluorophenyl) borates by reaction of boron trihalide with pentafluorophenyl lithium or pentafluorophenyl magnesium halide and by reaction of tris (pentafluorophenyl) boron with pentafluorophenyl lithium. See generally, published European patent application No. 0 604 961 A2 and U.S.

patent 5,473,036. Such borates are important intermediates in the synthesis of catalysts used in combination with metallocene polymerization catalysts.

SUMMARY OF THE INVENTION The invention provides a method for the production of tetrakis (pentafluorophenyl) borates by reacting a tetrafluoroborate salt with pentafluorophenyl magnesium halide in a non-interfering solvent.

DETAILED DESCRIPTION OF THE INVENTION Pentafluorophenyl magnesium halides (Grignards) are known. See, e.g., Respess, et al., J.Oraanometal.Chem.

(1969) 18:263-274 and Respess, et al., J.Orqanometal.

Chem. (1969) 19:191-195. Pentafluorophenyl magnesium halides useful in the invention have the formula C6F5MgX in which X is a halogen, e.g., iodine, chlorine or bromine, preferably bromine.

Tetrafluoroborate salts useful in the invention have the formula QBF4 in which Q is an sodium, lithium or potassium, preferably sodium.

The reaction between the Grignard and the tetrafluoroborate salt may be conducted in any non- interfering solvent. Typical solvents have the formula R-O-R' in which the R and R' are the same or different aliphatic hydrocarbon groups having 2-10 carbon atoms.

Ethyl ether is preferred. The reaction is appropriately

conducted at a temperature of 0°C to 100°C, preferably at 30 to 35"C.

The tetrafluoroborate salt is added to the Grignard solution in stoichiometric amounts or in such greater or lesser amount as may be deemed appropriate. In the preferred practice of the invention the tetrafluoroborate salt is added in an amount not less or more than 15% of stoichiometric. Preferably the reaction mixture is refluxed to expedite the reaction.

EXEMPLIFICATION OF THE INVENTION EXAMPLE I A reaction vessel is charged with 0.25 mol of C6H5MgBr and 0.056 mol of Na B F4 in an ethyl ether solvent. The reaction mixture is refluxed overnight under nitrogen. The reaction mixture was processed to recover of 24.4g of brown solids. Apparent yield 62.1%.

19F NMR peaks established the presence of tetrakis (pentafluorophenyl) borate in an amount corresponding to approximately 50% of the total product of the reaction.

EXAMPLE II Example I was repeated using Na BF4 ground by pestle and mortar and a 36 hour reflux time. Yield: 62.2% of a product having a tetrakis (pentafluorophenyl) borate purity of about 60% as indicated by 19F NMR.