Substituent, Solvent, and Dispersion Effects on the Zwitterionic Character and Dimerization Thermochemistry of the Group 6 Fulvene Metal Tricarbonyl Complexes

Dimerizations of fulvene metal tricarbonyl complexes of the type (C5H4CRR′)­M­(CO)3 (R, R′ = MeO, Me, H; M = Cr, Mo, W) to form a metal–metal bond and a new carbon–carbon bond, thereby giving binuclear cyclopentadienyl metal carbonyl derivatives, are predicted to be thermochemically favored but to have significant activation energies ranging from ΔE = 19 to 42 kcal/mol. However, the introduction of dimethyl­amino but not methoxy substituents onto the exocyclic carbon atom changes the situation drastically so that the monomers [C5H4CH­(NMe2)]­M­(CO)3 and [C5H4C­(NMe2)2]­M­(CO)3 become strongly thermochemically favored, lying ΔE = 43 kcal/mol (M = W) to 63 kcal/mol (M = Cr) below their corresponding dimers. In such dimethyl­amino-substituted (fulvene)­M­(CO)3 derivatives, the M–C distance to the exocyclic fulvene carbon is lengthened beyond the bonding distance to give a zwitterionic structure with a pentahapto fulvene ligand. Such M–C distances in (fulvene)­M­(CO)3 complexes, which have preferred zwitterionic structures, increase with increasing solvent polarity (i.e., dielectric constant) until a saturation point is reached.