Kinetic studies of group VI metal carbonyl complexes. IV. Substitution reactions of o-phenanthroline complexes of chromium hexacarbonyl

Article abstract of DOI:10.1021/ic50051a029

Cr(CO)₄(X-o-phen) reacts with P(OCH₂)₃CCH₃, L, at a rate which is independent of the concentration of L to form cis-Cr(CO)₃(L)(X-o-phen) as the only reaction product. The rate of reaction of Cr(CO)₄(X-o-phen) is however dependent on the basicity of the X-o-phen chelate in a manner which is predictable from a linear free energy expression. The reaction rate dependence on the pK_a of X-o-phen is contrary to what would be predicted from current π-bonding theories since the reaction rate was found to increase with increasing pK_a of the chelate. The C-O stretching frequencies of the Cr(CO)₄-(X-o-phen) complexes decrease with increasing basicity of X-o-phen. This would be predicted from π-bonding theory but does not explain the observed rate trend. The increases in reaction rates with increasing chelate pK_a are discussed in terms of a stabilized transition state and in ternis of labilizing and nonlabilizing ligands.