The effect of axial coordination and ligand strain on the multielectron-transfer chemistry of d 8d 8 rhodium isocyanide complexes: UV-vis-IR spectroelectrochemical studies

Abstract

The thermodynamic parameters governing the disproportionation reactions of a series of binuclear d 7-d 8 rhodium radicals are described. Complexes studied were of the general form Rh 2(diiso) 4-− x (dppm) x 3+ (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 d 7-d 8 tetrakis-diisocyanide species studied here is greatly favored. The d 7-d 8 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 d 8-d 8, d 7-d 8, and d 7-d 7 electronic states of the various compounds investigated. These studies show that the stabilities of the reduced d 8-d 8 and radical d 7-d 8 oxidation states remain relatively constant throughout the series. The large range of K disp (>10 12) encompassed by these compounds results from a high degree of variation in the relative energies of the d 7-d 7 2e − oxidation products. We demonstrate that differences in the strong affinity for bonding axial ligands by the electrochemically generated d 7-d 7 complexes are the main factor that determines K disp; however, the d 7-d 7 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 d 8d 8 and the dσ→ dσ ∗ excited state of the d 7d 7 species have geometries similar to those of the corresponding d 7-d 8 radical species.