SYNTHESIS, CHARACTERIZATION, AND REDOX PROPERTIES OF THE 17-VALENCE-ELECTRON COMPLEXES CP’CR(NO)(L)X

Treatment of CH2Cl2 solutions of [Cp’Cr(NO)I]2 [CP’= Cp (eta5-C5H5) or Cp* (eta5-C5Me5)] with various P- and N-containing Lewis bases, L, affords good yields of Cp’Cr(NO)(L)I complexes [L = PPh3, P(OMe)3, P(OPh)3, py, or C5H11N (pip)]. Photolysis of toluene solutions of CpCr(NO)(CO)2 in the presence of PPh3 and a source of halogen such as CH2Cl2 results in a 63% isolated yield of CpCr(NO)(PPh3)Cl. In donor solvents, S, the ESR-active CpCr(NO)(L)X (X = Cl, I) compounds undergo either L or X substitution to form the 17-valence-electron CpCr-(NO)(S)X and [CpCr(NO)(L)(S)+ species, respectively. A kinetic analysis of the reaction of CpCr(NO)(THF)I with PPh3 shows the process to be second-order overall (first-order in each reactant) and associative in nature. The reduction potentials for the various CpCr(NO)(L)X complexes in THF vary from -1.05 to -1.65 V. These reductions are irreversible and are followed by loss of X-. Consistently, reduction of CpCr(NO)(PPh3)I in THF by Zn in the presence of P(OPh)3 affords the 18-valence-electron mixed phosphine-phosphite complex, CpCr(NO)(PPh3)[P(OPh)3]. The redox chemistry of the CpCr(NO)(L)X complexes is thus unusual in that the odd-electron species are substitutionally less labile than are, their 18-electron reduction products.