CO Substitution in Cyclopentadienyl Group V Carbonyl Compounds. A Reexamination of the Kinetics of (h⁵-C₅H₅)M(CO)₄, where M = Nb and Ta. Dinara Lukmanova, Tom Bitterwolf, and Skip Gallagher. Department of Chemistry, University of Idaho, Moscow, ID 83844-2343. The synthesis of a large family of ring-substituted compounds of the general form CpM(CO)₄, where M = Nb and Ta, as well as the analogous indenyl compounds has provided an opportunity to examine in detail the effect of ring-substituents on the rates of carbon monoxide exchange. Previous work (J. W. Freeman and F. Basolo, Organometallics, 1991, 10, 256-263) has indicated that the reaction:

CpNb(CO)₄ + L --------> CpNb(CO)₃L + CO

(where L = phosphine) proceeds by both associative and dissociative routes. A reexamination of these reaction kinetics including relatively low ligand to metal complex ratios suggests that the actual mechanism is purely dissociative in nature.