Dimerizations of fulvene metal tricarbonyl complexes of the type (CHCRR')M(CO) (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 Δ = 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 [CHCH(NMe)]M(CO) and [CHC(NMe)]M(CO) become strongly thermochemically favored, lying Δ = 43 kcal/mol (M = W) to 63 kcal/mol (M = Cr) below their corresponding dimers. In such dimethylamino-substituted (fulvene)M(CO) 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) complexes, which have preferred zwitterionic structures, increase with increasing solvent polarity (i.e., dielectric constant) until a saturation point is reached.
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Dimerizations of fulvene metal tricarbonyl complexes of the type (CHCRR')M(CO) (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 Δ = 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 [CHCH(NMe)]M(CO) and [CHC(NMe)]M(CO) become strongly thermochemically favored, lying Δ = 43 kcal/mol (M = W) to 63 kcal/mol (M = Cr) below their corresponding dimers. In such dimethylamino-substituted (fulvene)M(CO) 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.
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