COORDINATION CHEMISTRY OF COMPLEXES WITH METAL-METAL MULTIPLE BONDS -REVERSIBLE COORDINATION OF CYANIDE ION BY DIMOLYBDENUM AND DITUNGSTENHEXAALKOXIDES

The dimetal-hexaalkoxides of molybdenum and tungsten (M=M) [1a: W2(O(t )Bu)6, 1b: W2(OCH2(t)Bu)6, 1c: Mo2(O(t)Bu)6, 1d: Mo2(O(i)Pr)6, 1e: Mo2 (OCH2(t)Bu)6] react reversibly with one equivalent of cyanide ion as i ts (n)Bu4N salt in non-polar media to form mono-adducts of formula [nB u4N]+[M2(OR)6(CN)]- (2a-e). The spectroscopic data are consistent with the presence of a mu-CN moiety which interacts predominantly as a sig ma-donor, but do not discount alternative structures which exhibit rap id fluxionality. Addition of a second equivalent (or more) of nBu4NCN leads to the reversible formation of 1,2-di-adducts of formula [nBu4N] 2+[M2(OR)6(CN)2]2- (3a, 3b, 3d, 3e). Coordination of a second equivale nt of cyanide is significantly less favorable than the first so that t he consecutive equilibria may be measured. Determination of K(eq) for the equilibriUM M2(OR)6+CN- half arrow right over half arrow left [M2( OR)6(CN)]- allows the first quantitative comparison of the Lewis acidi ty of the reactive dimetal hexaalkoxides. The identity of the metal an d steric factors have a significant effect as shown by the following d ata: for the formation of 2a, DELTAH-degrees = -11.2(l) kcal mol-1, DE LTAS-degrees = - 18.6(7) eu; 2c, DELTAH-degrees = - 8.8(l) kcal mol-1, DELTAS-degrees = - 18.8(9) eu; 2d, DELTAH-degrees = - 10.5(1) kcal mo l-1, DELTAS-degrees = - 18.0(8) eu. Complexes 2b and 2e showed no sign s of dissociation under similar conditions and thermodynamic parameter s associated with their formation may only be estimated (DELTAH-degree s = - 16.9 kcal mol-1, DELTAS-degrees = - 18.5 eu and DELTAH-degrees = - 13.5 kcal mol-1, DELTAS-degrees = - 18.5 eu respectively). At high temperatures the equilibrium becomes fast relative to the NMR time sca le for molybdenum, and this allows an estimation of the rate of dissoc iation by dynamic lineshape analysis. For [Mo2(0(t)BU)6 (CN)]- (2c) DE LTAH(double dagger) = 22.0(5) kcal mol-1, DELTAS(double dagger) = 13.5 (8) eu while for [Mo2(OCH2(t)Bu)6 (CN)]- (2e) DELTAH(double dagger) = 22.2(5) kcal mol-1, DELTAS(double dagger) = 13.9(8) eu. Line broadenin g was not observed for the tungsten alkoxide complexes 2a-b consistent with the greater thermodynamic strength of the W2(CN) bond (DELTADELT AH-degrees = - 2.4 kcal mol-1). The second equilibrium [M2(OR)6(mu-CN) ]- + CN- half arrow right over half arrow over left [M2(OR)6(CN)2]2- s hows a similar dependence on the nature of the metal and alkoxide: for formation of 3a, DELTAH-degrees = - 9.3(5) kcal mol-1, DELTAS-degrees = - 29(2) eu; for 3e, DELTAH-degrees = - 11.6(5) kcal mol-1, DELTAS-d egrees = - 29(2) eu. In contrast, 3c was not detected under any condit ions (DELTAH-degrees = - 6.9 kcal mol-1, DELTAS-degrees = - 29.2 eu, e stimated) while 3b was undissociated under similar conditions (DELTAH- degrees = 15.0 kcal mol-1, DELTAS-degrees = - 29.2 eu, estimated).