Ralph Schenker, Høgni Weihe and Hans U. Güdel
Exchange Interactions Derived from Electron Transfer Processes in [Cr₂(OH)₃(tmtame)₂](NO₃)₃
Inorg. Chem. 38, 5593-5601 (1999)                 

Abstract: From polarized optical absorption and emission spectra of [Cr₂(OH)₃(tmtame)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 ⁴A₂⁴A₂ ground as well as the ²E⁴A₂ and ²T₁⁴A₂ singly and ²E²E, ²E²T₁, ²T₁²T₁, ²E²T₂, and ²T₁²T₂ doubly excited states of the ground electron configuration are determined, the latter 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 single-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 splittings are rationalized using a model based on a valence bond approach (VBCI), where the exchange interactions are derived from configuration interaction of LMCT and metal-to-metal charge transfer (MMCT) electron configurations with the ground configuration. The splittings are well reproduced by this model over a range of about 40000 cm^–1. Trigonal orbital exchange parameters 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 situation in the LMCT configuration, leading to the observed ferromagnetic energy ordering of LMCT levels. Magnetostructural correlations are established from a comparison of the title compound with the similar complex [Cr₂(OH)₃(tmtacn)₂](ClO₄)₃ (tmtacn = 1,4,7-trimethyl-1,4,7-triazacyclononane).