Reaction Mechanisms of Organometallic Systems

The metal carbonyl family of compounds is typical of the range of Structures and reactivities of organometallic complexes. The rate of CO exchange was examined in early studies, and this work is the subject of a recent review. The order of reaction rates is as follows  

Where the rate law has been determined, the reaction is first-order in [M(CO)J and zero-order in [CO]. This implies a D mechanism, since a solvent intermediate is unlikely for the noncoordinating solvents. This mechanism also is probable for other ligand substitutions. 

The main mechanistic exception to the above generalizations is V(CO)g, which has an I, mechanism for PRj substitution reactions. This compound is unique in that it is the only 17-electron metal carbonyl and also is by far the most labile. Some kinetic results for substitution on V(CO)g in hexane are given in Table 5.1. 

The substitution rates have rather low AH values, and the negative AS values are typical of an associative process. The rates for various entering groups correlate with the basicity rather than the size, as measured by the cone angle. It has been suggested that formation of a 19-electron associative intermediate from a 17-electron reactant is much more favorable than a 20-electron intermediate from an 18-electron reactant. 

Ziegler indicates that relativistic effects contribute about 3 and 10 kcal mol to strengthen the bonds in Ru(CO)j and Os(CO)j, respectively, and this difference largely accounts for the lower reactivity of OsfCO) . There is an implication that the relativistic effects are somewhat less important in the M(C0)4 product than in the M(CO)j reactant. 

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