METAL-METAL BONDING IN PENTAMETHYLCYCLOPENTADIENYLMOLYBDENUM(IV) DINUCLEAR COMPOUNDS - CHLORIDE ABSTRACTION FROM NONBONDED CP-ASTERISK-2MO2CL6 TO AFFORD BONDED [CP-ASTERISK-2MO2CL5]

Chloride abstraction from Cp*2Mo2Cl6 (Cp*=eta(5)-C(5)Me(5)) is accomplished by interaction with the Lewis acid AlCl3 to afford the structurally characterized salt [Cp*2Mo2Cl5](+)[AlCl4](-). Crystal data: triclinic, space group P (1) over bar, a=8.3903(13), b=15.797(3), c=24.036(2) Angstrom, alpha=86.766(11), beta=80.916(10), gamma=81.616(14)degrees, V=3110.5(8) Angstrom(3), D-c=1.726 Mg m(-3), mu(Mo K alpha)=1.618 mm(-1), R=0.0637. The structure exhibits two four-legged piano stools joined by three bridging Cl atoms. The Mo-Mo distance of 2.866(2) Angstrom is significantly longer than all other reported bonded Mo(IV)-Mo(IV) distances and longer than the single bond (sigma(2) delta(*2)delta(2)) distance in the related Mo(III) complexes [(ring)MoCl2](2) (ring=substitute cyclopentadienyl ring). The reasons for this lengthening are analyzed and discussed on the basis of structural data and Fenske-Hall MO calculations. [Cp*2Mo2Cl5](+) reacts rapidly with Cl- to afford [Cp*MoCl4](-) and exhibits a reversible one-electron oxidation to a neutral Cp*2Mo2Cl5 species at -0.13 V versus ferrocene/ferricinium. The non-existence of a metal-metal bonded isomeric form of the Cp*2Mo2Cl5 parent compound is also discussed.