Exchange interactions derived from electron-transfer processes in [Cr-2(OH)(3)(tmtame)(2)](NO3)(3)

From polarized optical absorption and emission spectra of [Cr-2(OH)(3)(tmta me)(2)](NO3)(3) (tmtame = N,N',N "-trimethyl-1,1,1 -tris(aminomethyl)ethane ) in the visible and near UV, the exchange splittings of the (4)A(2)(4)A(2) ground as well as the (2)E(4)A(2) and (2)T(1)(4)A(2) singly and (EE)-E-2-E -2, (ET1)-E-2-T-2. (T1T1)-T-2-T-2, (ET2)-E-2-T-2, and (T1T2)-T-2-T-2 doubly excited states of the ground electron configuration are determined, the la tter corresponding to simultaneous pair excitations by a single photon. The bulk of intensity in the region of these doubly excited states is found to be vibronically induced by an electric-dipole exchange mechanism. From sin gle-crystal and glass absorption spectra a ferromagnetic energy ordering of the lowest energy Ligand-to-metal charge transfer (LMCT) states is derived , whereas the ground state is antiferromagnetically split. The observed spl ittings are rationalized using a model based on a valence bond approach (VB CI), where the exchange interactions are derived from configuration interac tion of LMCT and metal-to-metal charge transfer (MMCT) electron configurati ons with the ground configuration. The splittings are well reproduced by th is model over a range of about 40 000 cm(-1). Trigonal orbital exchange par ameters J(a) and J(e) are derived, revealing that the direct pathway along the Cr-Cr axis is the dominant one. This gives rise to a double exchange si tuation in the LMCT configuration, leading to the observed ferromagnetic en ergy ordering of LMCT levels. Magnetostructural correlations are establishe d from a comparison of the title compound with the similar complex [Cr-2(OH )(3)(tmtacn)(2)] (ClO4)(3) (tmtacn = 1,4,7-trimethyl-1,4,7-triazacyclononan e).