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(SiMe2CH 2PPh2)(2), N(SiMe2-(CH2PPr2)-Pr-i)(2); COE = cyclooctene) with a serie s of phosphine ligands and CO have been measured by solution calorimet ry. The measured enthalpies span range of ca. 40 kcal/mol. These syste ms favor coordination of strong pi-acceptor/weak sigma-donor ligands a s 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 o pposite of that observed in another square planar rhodium(I); system, trans-RhCl(CO)(PZ(3))(2). With the exception of CO, the ligands invest igated are isosteric, and so the observed trends are electronic in nat ure. Single-crystal X-ray diffraction studies on several of theses com plexes ([RPNP]RhL where R, L = Ph, PPh3; Ph, Ppyrl(3); Ph, CO; Pr-i, P Ph3; Pr-i, Ppyrl(3); Pr-i, CO; Pr-i, COE) have been performed. Althoug h the structural trends are readily understood in:terms of the electro nic (donor/acceptor) nature of each ligand array, it is not obvious th at the structural data predict the trends or, in particular, the trend reversal in Delta H-rxn in the two Rh(I) systems. Rather, these resul ts illustrate the importance of reorganization energies in thermodynam ic analyses of metal-ligand bonding, especially in the presence of syn ergistic bonding involving sigma-donor, pi-donor, and pi-acceptor liga nds, interacting through shared metal orbitals (electron push-pull). I n such cases the interpretation of a metal-ligand bond dissociation en thalpy (D) as an intrinsic, universal, and transferable property of th at bond (e.g., a ''bond strength'') is an invalid proposition.