Properties of Metal-Metal Single Bonds. Vibrational and Electronic Spectra of Binuclear Rhodium(II) and Iridium(II) Isocyanide Complexes with Comparisons to Mn2(CO)10

Article abstract of DOI:10.1021/ja00312a021

Vibrational and electronic spectra of Rh2b4L2(n+), Rh2(TMB)4L2(n+) (b = 1,3-diisocyanopropane; TMB = 2,5-dimethyl-2,5-diisocyanohexane; L = H2O, CH3CN (n = 4); L = Cl, Br, I, N3, NCS (n= 2)), and Ir2(TMB)4L2(2+) (L = Cl, Br, I) have been measured.The three stretching vibrations of the linear M2L2 core are assigned and the M2 force constants are shown to be ca. 0.9-1.6 mdyn/Angstroem.The electronic spectra are dominated by an intense (oscillator strength 0.6-1.0) d?->d?^* transition.Variation in the maximum of this band over the range 33000 to 24000 cm^-1 (Rh2(4+)) or 37000 to 30000 cm^-1 (Ir2(4+)) as a function of the axial ligand is attributed to mixing with an L->d?^* charge transfer transition.The temperature dependences of the d?->d?^* transitions of the Rh2(4+) complexes and Mn2(CO)10 have been analyzed by the method of moments and shown to be consistent with a large excited state distortion along the metal-metal stretching coordinate.A weaker absorption near 24000 cm^-1 is shown from low temperature single crystal polarized spectra of (PF6)2 to contain the d?^*->d?^* and dδ^*->d?^* excitations.Other weak bands of the Rh(II) complexes near 33000 and 43000 cm^-1 are tentatively assigned to d?^*,dδ^*->d?^* and d?^*->d_x2_-y2 transitions, respectively.The photochemistry of the binuclear Rh(II) complexes is shown to be consistent with halogen radical release from the halide complexes.The excited state lifetime is less than 20 ps.