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 Kdisp (>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 Kdisp; 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.
Bibliographical noteFunding Information:
We would like to acknowledge the continuing moral support of Harry B. Gray, one tremendously gracious human being. Work at the University of Minnesota was supported by the National Science Foundation. We thank Johnson-Matthey, Inc. for generous loans of rhodium trichloride.
- Electron transfer
- Isocyanide complexes
- Rhodium complexes