Computational assessment of the effect of sigma-pi bonding synergy and reorganization energies on experimental trends in rhodium-phosphine bond enthalpies

Via a series of systematic density functional (B3LYP/LANL2DZ) computational experiments, we have examined the origin of opposing Rh-PR3 bond enthalpy trends involving two different square-planar Rh(I) complexes with a series of different pi -accepting phosphines (Huang, J. K.; Haar, C. M.; Nolan, S. P.; Marshall, W. J.; Moloy, K. G. J. Am. Chem. Sec. 1998, 120, 7806). Comp utational results rule out reorganization energies as the cause of the cont radictory trends in thermodynamic analyses of metal-ligand bonding. Rather, calculations show that synergy of sigma -donor and pi -acceptor ligands li nked to a metal is pivotal for interpreting the contradictory trends in Rh- P bond enthalpies. We conclude that metal-phosphine bond energies cannot be regarded as intrinsic, universal, or transferable.