Thermodynamics of phosphine coordination to the [PNP]Rh(I) fragment: An example of the importance of reorganization energies in the assessment of metal-ligand 'bond strengths'

Article abstract of DOI:10.1021/ja974200v

Reaction enthalpies of the complexes [RPNP]Rh(COE) ([RPNP] = N(SiMe₂CH₂PPh₂)₂, N(SiMe₂-CH₂P(i)Pr₂)₂; COE = cyclooctene) with a series of phosphine ligands and CO have been measured by solution calorimetry. The measured enthalpies span a range of ca. 40 kcal/mol. These systems favor coordination of strong π-acceptor/weak σ-donor ligands as shown by the trend in ΔH(rxn): CO >> Ppyrl'₃ > Ppyrl₃ > PPhpyrl₂ > PPh₂pyrl > PPh₃. This trend is exactly the opposite of that observed in another square planar rhodium(I) system, trans-RhCl(CO)(PZ₃)₂. With the exception of CO, the ligands investigated are isosteric, and so the observed trends are electronic in nature. Single-crystal X-ray diffraction studies on several of theses complexes ([RPNP]RhL where R, L = Ph, PPh₃; Ph, Ppyrl₃; Ph, CO; (i)Pr, PPh₃; (i)Pr, Ppyrl₃; (i)Pr, CO; (i)Pr, COE) have been performed. Although the structural trends are readily understood in terms of the electronic (donor/acceptor) nature of each ligand array, it is not obvious that the structural data predict the trends or, in particular, the trend reversal in ΔH(rxn) in the two Rh(I) systems. Rather, these results illustrate the importance of reorganization energies in thermodynamic analyses of metal-ligand bonding, especially in the presence of synergistic bonding involving σ-donor, π-donor, and π-acceptor ligands, interacting through shared metal orbitals (electron push-pull). In such cases the interpretation of a metal-ligand bond dissociation enthalpy (D) as an intrinsic, universal, and transferable property of that bond (e.g., a 'bond strength') is an invalid proposition.