When aryl groups are substituents in the 1,4-positions of the diazabutadiene (DAB) ligand,2 substitution of carbon monoxide by PMes in Fe(CO)s(DAB) takes place solely by a second-order process. The rate law is first order in both Fe(CO)s(DAB) and PMes. Activation parameters for the 4-fluorophenyl derivative in toluene support the associative nature of this reaction: △H*=13.6±1.0kcal/mol; △S*=-34.8±3.2 eu. Carbon monoxide replacement rates depend on the nature of the nucleophile, and increase in the series PPh3<P(OMe)3<P(n-Bu)3<PMe3. This rate also increases when the π-acceptor ability of the DAB ligand increases. When bulky tert-butyl groups are the substituents in the 1,4-positions of the DAB ligand, steric interactions become important in the six-coordinate transition state. Nucleophilic attack on this complex results in loss of the DAB ligand to give Fe(CO)3(PMe3)2. This reaction obeys a two-term rate law involving both associative (△H*=20.4±0.8kcal/mol and △S*=-18.2±2.0 eu) and dissociative (△H*=25.0±1.0kcal/mol and △S*=-8.0±3,0 eu) pathways. Factors which facilitate nucleophilic attack on iron in these coordinately saturated metallacycles and in the related Fe(CO)3(N4Me2) complex are discussed.

Kinetic data are reported for the reactions of Mo(CO)5L (where L1=P(c-Hx)3, P(n-Bu)3, NMe3, py, PPh3, AsPh3, P(OEt)3, or P(OMe)3) with L in the presence of Me3NO to form cis-Mo(CO)4L2. The rates of reactions are second order: first order in Mo(CO)5L concentration, first order in Me3NO concentration, and zero order in L concentration. For ligand L with cone angles less than 135*, the rates of reaction increase with increasing stretching frequency of the CO bands in the IR. This supports the proposed mechanism, which involves attack by the O atom of Me3NO on a C of a CO cis to L in Mo(CO)sL. For L=PPh3 or AsPh3, the reactions are faster than expected on the basis of their vco values, and this is discussed in terms of steric effects.

Vanadium hexacarbonyl readily disproportionates upon treatment with oxygen and nitrogen Lewis bases. The reaction is first order with respect to Lewis base and V(CO)6. Nucleophilic attack on the metal center appears to be the rate-determining step. Second-order rate constants in dichloromethane decrease in the series py>Et3N>MeCN>MeOH>acetone>THF>2,5-Me2THF>DME>MeNO2>EhO, with a factor of 104 separating the first and last members of this group. Activation parameters for disproportionation by THF are in accord with an associative mechanism: △H*=14.2±1.2 kcal/mol and △S*=-21.5±4.2 cal/mol.deg. The structure of the disproportionation product is also dependent on the nature of the Lewis base. For Et20, the bridging isocarbonyl complex [V(Et20)4] [O-C-V(CO)5]2 can be isolated from CH2C12-Et20 solution. For stronger oxygen and nitrogen bases (B), [V(B)6] [V(CO)6]2 is the final product. In the case of B = pyridine, a bridging isocarbonyl intermediate can be detected as a kinetic product of the disproportionation process. This intermediate reacts with additional pyridine to afford [V(B)6] [V(CO)6]2. The observation of an isocarbonyl-bridged intermediate suggests that electron transfer may take place through an isocarbonyl ligand. Phosphine-substituted derivatives of V(CO)6 undergo disproportionation much more slowly than V(CO)6, although the rate-limiting step also appears to be CO substitution by the Lewis base. For example, disproportionation of V(CO)5P(n-Bu)3 induced by CH3CN is five orders of magnitude slower than that of V(CO)6.