Time-resolved resonance Raman spectroscopy at low temperature.: The excited-state metal-metal stretching frequency of Rh2(TMB)42+ (TMB = 2,5-dimethyl-2,5-diisocyanohexane)

The time-resolved resonance Raman spectrum of the short-lived triplet (d sigma*p sigma) excited state of Rh-2(TMB)(4)(2+) (TMB = 2,5-dimethyl-2,5-diisocyanohexane) was obtained by lowering the temperature of a 3:1 ethanol/methanol solution until the excited-state lifetime became much greater than the width of the pulsed laser excitation source. The metal-metal stretching frequency is 151 cm(-1) in the excited triplet state, as compared to 50 cm(-1) in the ground state. The diatomic harmonic force constants derived from these frequencies are in a 9.12:1 ratio (excited state/ground state), consistent with the simple molecular orbital description that predicts that the Rh-Rh bond order is greater in the excited state than in the ground state. A comparison of Rh2(TMB)42+ and Rh(2)b(4)(2+) (b = 1,3-diisocyanopropane) Raman data indicates that the nature of the bridging ligand considerably affects the ground- and excited-state metal-metal stretching frequencies and that the population of the p sigma orbital may have very little effect on the bonding in the excited triplet state.