The effect of axial coordination and ligand strain on the multielectron-transfer chemistry of d⁸d⁸ rhodium isocyanide complexes: UV-vis-IR spectroelectrochemical studies

The thermodynamic parameters governing the disproportionation reactions of a series of binuclear d⁷-d⁸ rhodium radicals are described. Complexes studied were of the general form Rh₂(diiso)_4-x(dppm)_x³⁺ (where diiso = 2,5-diisocyano-2,5-dimethylhexane (TM4) and x = 0 and 2,1,8-diisocyanomenthane (dimen) and x = 0-2, and 1,3-diisocyanopropane (bridge) and x = 2; dppm= 1,1′-bis(diphenylphosphino)methane). Disproportionation of the d⁷-d⁸ tetrakis-diisocyanide species studied here is greatly favored. The d⁷-d⁸ oxidation state of the dppm substituted analogs do not appreciably disproportionate. Variable-temperature UV-vis and IR spectroelectrochemical experiments have shown that the diminished propensity for bis-dppm complexes to disproportionate originates from large, unfavorable enthalpic and entropic terms that accompany the replacement of diisocyanide ligands by dppm. To account for this behavior, equilibrium measurements have been carried out to establish the relative stabilities of the d⁸-d⁸, d⁷-d⁸, and d⁷-d⁷ electronic states of the various compounds investigated. These studies show that the stabilities of the reduced d⁸-d⁸ and radical d⁷-d⁸ oxidation states remain relatively constant throughout the series. The large range of K_disp (>10¹²) encompassed by these compounds results from a high degree of variation in the relative energies of the d⁷-d⁷ 2e^- oxidation products. We demonstrate that differences in the strong affinity for bonding axial ligands by the electrochemically generated d⁷-d⁷ complexes are the main factor that determines K_disp; however, the d⁷-d⁷ complexes that contain the dimen ligand are significantly destabilized by increases in ligand strain energy relative to the TM4 compounds. Finally, we have identified correlations between the differences in ground state complexation energies and electronic spectroscopy data that suggest the dσ*→ pσ excited state of the d⁸d⁸ and the dσ→ dσ* excited state of the d⁷d⁷ species have geometries similar to those of the corresponding d⁷-d⁸ radical species.