ELECTRON PAIRING AS A STABILIZING FACTOR IN OPEN-SHELL ORGANOMETALLICS - THE CASE OF 15-ELECTRON CPMCL(2)(PH3) (M=CR AND MO)

DFT (BLYP) calculations with full geometry optimization on the PH3 add ition to CpMCl(2)(PH3) show very different results for M = Cr and Mo, in accord with the experimentally established stability of 15-electron CpCrX(2)L and 17-electron CpMoX(2)L(2) (X = 1-electron ligand; L = 2- electron ligand). The calculations point to the paramount importance o f electron pairing energy: 22.53 kcal/mol must be spent to promote the ground state (4)A '' CpCrCl(2)(PH3) to the (2)A' excited state, and o nly 16.51 kcal/mol are regained upon formation of the second Cr-PH, bo nd. The cost of pairing the electrons in CpMoCl(2)(PH3), on the other hand, is much less (3.95 kcal/mol) and the bond formation energy along the spin doublet surface is 12.26 kcal/mol. The relevance of electron pairing energy as a stabilizing factor for the general class of open- shell organometallics is pointed out.