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

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 its (n)Bu4N salt in non-polar media to form mono-adducts of formula [nBu4N]+[M2(OR)6(CN)]- (2a-e). The spectroscopic data are consistent with the presence of a mu-CN moiety which interacts predominantly as a sigma-donor, but do not discount alternative structures which exhibit rapid 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 equivalent of cyanide is significantly less favorable than the first so that the 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 acidity of the reactive dimetal hexaalkoxides. The identity of the metal and steric factors have a significant effect as shown by the following data: for the formation of 2a, DELTAH-degrees = -11.2(l) kcal mol-1, DELTAS-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 mol-1, DELTAS-degrees = - 18.0(8) eu. Complexes 2b and 2e showed no signs of dissociation under similar conditions and thermodynamic parameters associated with their formation may only be estimated (DELTAH-degrees = - 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 scale for molybdenum, and this allows an estimation of the rate of dissociation by dynamic lineshape analysis. For [Mo2(0(t)BU)6 (CN)]- (2c) DELTAH(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 broadening was not observed for the tungsten alkoxide complexes 2a-b consistent with the greater thermodynamic strength of the W2(CN) bond (DELTADELTAH-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- shows 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-degrees = - 29(2) eu. In contrast, 3c was not detected under any conditions (DELTAH-degrees = - 6.9 kcal mol-1, DELTAS-degrees = - 29.2 eu, estimated) while 3b was undissociated under similar conditions (DELTAH-degrees = 15.0 kcal mol-1, DELTAS-degrees = - 29.2 eu, estimated).