Exchange Interactions Derived from Electron-Transfer Processes in [Cr(2)(OH)(3)(tmtame)(2)](NO(3))(3).

Abstract

From polarized optical absorption and emission spectra of [Cr(2)(OH)(3)(tmtame)(2)](NO(3))(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 (2)E(2)E, (2)E(2)T(1), (2)T(1)(2)T(1), (2)E(2)T(2), and (2)T(1)(2)T(2) 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 40 000 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 [Cr(2)(OH)(3)(tmtacn)(2)](ClO(4))(3) (tmtacn = 1,4,7-trimethyl-1,4,7-triazacyclononane).