Transition-metal-catalyzed C-C cross-coupling reactions involving sp³-, sp²- and sp- carbon atoms are well established as a practical tool of synthetic organic chemistry. However the factors controlling the key C-C bond formation step remain poorly understood. We have synthesized nine complexes of the type (PCP)Ir(R)(R') (PCP = k³-C₆H₃-2,6-(CH₂P^tBu₂)₂) (where R, R' = Ph, Me, CH=CHPh or CCPh) and determined the activation energies for C-C elimination, both experimentally and theoretically. The complexes have all been crystallographically characterized as the corresponding six-coordinate CO adducts. It has been previously proposed that an important factor favoring the rate of C-C reductive elimination is greater s character in the C-based orbital used to bind to the metal in complexes of the type R₂M(PH₃)₂ (where M = Pd, Pt) (e.g. sp² more favorable than sp³). Our results, however indicate that complexes of the type (PCP)Ir(Me)(R) and (PCP)Ir(CH=CHPh)(R), have similar activation barriers, while complexes of the type (PCP)Ir(Ph)(R) have much higher barriers. DFT calculations have shown that the vinyl or phenyl groups must rotate in the transition state which involves the pi system of these ligands. It appears that the ligand geometry and the availability of the pi system rather than the lesser directionality of the sp² orbitals, is the major factor facilitating reductive elimination of sp²- and sp-carbon-based ligands.

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