ELECTRONIC ABSORPTION AND MCD SPECTRA OF M2(TMB)4(2+), M = RH AND IR - A VALENCE-BOND DESCRIPTION OF THE UPPER ELECTRONIC EXCITED-STATES

Electronic absorption and magnetic circular dichroism (MCD) spectra of Rh2(TMB)42+ and Ir2(TMB)42+ are reported along with polarized single-crystal absorption spectra of [Ir2(TMB)4][B(C6H5)4]2·CH3C6H5 (TMB = 2,5-diisocyano-2,5-dimethylhexane). Interpretation of the spectra is based on a valence-bond model that accommodates highly perturbed dimer transitions as well as monomer-like dimer excitations. In this model, half of the dimer electronic excited states possess ionic character; these states involve metal-to-metal charge transfer (MMCT). The intense absorptions of Ir2(TMB)42+ are assigned as follows: dσ* → pσ [A1g(1A1g)) → Eu(3A2u) and A2u(1A2u)] at 820 and 635 nm; dxzyz → pz [A1g (1A1g) → Eu, A2u(3Eu) and Eu(1Eu)] at 373, 331, and 320 nm; and dxy → pz [A1g(1A1g) → Eu(3B1u)] at 304 nm. Several weak features are observed to lower energy of the A1g(1Alg) → Eu(3A2u) band. The most prominent of the weak features (~430 nm) is assigned to the transition to1A1g (a single-center dz2 → pz excitation). High-energy features (λ < 300 nm) in the spectra of Rh2(TMB)42+ and Ir2(TMB)42+ are assigned to MMCT arising from dxz,yz → pz excitations. © 1990, American Chemical Society. All rights reserved.