The complex V(CO)5(NO) undergoes substitution of CO at or below 0℃ by L=PMe3, PPh3, P(O-i-Pr)3, P(OMe)3, and NEt3 to yield V(CO)4L(NO). Substitution proceeds according to a two-term rate law: -d[V(CO)5(NO)]/dt=kl[V(CO)5(NO)]+k2[V(CO)5(NO)] [L]. For the extremely weak NEt3 nucleophile, the substitution reaction occurs only via the k1 path. Substitution of a second CO ligand to afford V(CO)3(PMe3)2(NO) proceeds solely by a dissociative pathway and is 106 times slower than loss of CO from V(CO)5(NO) at 25℃. The thermal instability of V(CO)5(NO) results from facile CO dissociation at room temperature, which occurs faster than in any other first-row-metal carbonyl or carbonyl nitrosyl. This remarkable reactivity of V(CO)5(NO) may be attributed to a trans effect from the NO ligand. Ground-state SCF-Xa-DV calculations have been performed for V(CO)5(NO) and compared with those for V(CO)6. In V(CO)5(NO), the fully occupied r-bonding t2s orbitals split into b2(dxy) and e(dxz,yz) orbitals. The e level, which can π-bond to NO, is preferentially stabilized, and the NO π-orbital contribution is 4-5 times that of the axial CO ligand. This superior 7r-acceptor character of NO also leads to the purple color of V(CO)5(NO). One-electron transitions from the b2(dxy) and e(dxz,yz) levels into a low-lying empty NO π* orbital are calculated at 2.29 and 2.52 eV and observed experimentally at 2.23 and 3.14 eV, respectively.

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.

Carbon monoxide substitution in the metal radical V(CO)6 proceeds at or below room temperature to form monosubstitution products V(CO)5L (L=phosphine or phosphite). The substitution occurs solely by a second-order process according to a rate law that is first order in both V(CO)6 and phosphorus nucleophile. The rate of reaction is strongly dependent on the basicity and size of the ligand. Activation parameters further support the associative nature of the reaction: P(n-Bu)3, △H*=7.6 4±0.4 kcal/mol, △S*=-25.2±1.7cal/mol.deg; P(OMe)3, △H*=10.9±0.2kcal/mol, △S*=-22.6±0.8cal/mol.deg; PPh3, △H*=10.0±0.4kcal/mol, △S*=-27.8±1.6cal/mol.deg. The rate of substitution of V(CO)6 by PPh3 is unchanged under 1 atm of carbon monoxide or in the presence of [V(CO)6]-. The carbon monoxide substitution reactions of V(CO)sL with additional L also proceed by an associative mechanism with the rate of substitution approximately three orders of magnitude slower than for V(CO)6. The disubstituted product adopts the cis stereochemistry with small phosphorus donor ligands or with chelating phosphines. For L=P(OMe)3, activation parameters were determined: △H*=13.2±0.4kcal/mol, △S*=-27.6±1.8 cal/mol.deg. Phosphine exchange reactions of V(CO)sL were also observed indicating that, in addition to carbon monoxide, phosphine ligands on vanadium are substitution labile. Nucleophilic attack of P(n-Bu)3 at V(CO)5[P(n-Bu)3] is l05 times slower than that at V(CO)6, presumably because the increased electron density on the metal hinders nucleophilic attack. Quantitative comparisons between the 17-electron complex V(CO)6 and its 18-electron analogue Cr(CO)6 indicate that associative carbon monoxide substitution takes place 1010 times faster in the vanadium system.

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.