REDOX EQUILIBRIA OF THE NITROSONIUM CATION AND OF ITS NONBONDED COMPLEXES

The coordinatively unsaturated nitrosonium cation is reduced at the reversible redox potential of E° = 1.28 V vs SCE, as determined by the cyclic voltammetry of either the salt NO+BF4− on the initial negative scan or nitric oxide on the initial positive scan in acetonitrile. The marked solvent dependence of E° in other aprotic media (such as dichloromethane and nitromethane) correlates linearly with Gutmann‧s DN parameter for solvent donicity. Solvation as the critical factor in the stabilization of NO+ is akin to its formation of charge-transfer complexes with various aromatic donors. In the latter case, the analytical solution of the electrode kinetics for NO+ in the presence of arenes (Ar) by digital simulation of the cyclic voltammograms yields the reversible reduction potentials of a series of 1:1 complexes [NO+,Ar], in which Ec° also varies linearly with the oxidation potential of the arene ligand. This, coupled with the notable 260-mV shift in the nitrosonium reduction potential to Ec° = 1.02 V (for Ar = hexamethylbenzene), underscores the importance of electron donation in the stabilization of NO+ by nonbonded ligands. The rate constant for NO+ reduction is 100 times slower than that for the related dioxygen/superoxide couple. The relatively large reorganization energy for electron transfer is readily identified with the force constant and bond distance change inherent to this simple diatomic redox couple, and it accords with the theoretical predictions of Marcus. © 1990, American Chemical Society. All rights reserved.