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'

Reaction enthalpies of the complexes [RPNP]Rh(COE) ([RPNP] = N(SiMe2CH2PPh2)(2), N(SiMe2-(CH2PPr2)-Pr-i)(2); COE = cyclooctene) with a series of phosphine ligands and CO have been measured by solution calorimetry. The measured enthalpies span range of ca. 40 kcal/mol. These systems favor coordination of strong pi-acceptor/weak sigma-donor ligands as shown by the trend in Delta H-rxn: CO much greater than Ppyrl(3)' > Ppyrl(3) > PPhpyrl(2) > PPh(2)pyrl > PPh3. This trend is exactly the opposite of that observed in another square planar rhodium(I); system, trans-RhCl(CO)(PZ(3))(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, Ppyrl(3); Ph, CO; Pr-i, PPh3; Pr-i, Ppyrl(3); Pr-i, CO; Pr-i, 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 Delta H-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 sigma-donor, pi-donor, and pi-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.

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