The electronic absorption and magnetic circular dichroism spectra of IrBr62-in several host crystals

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Abstract
The absorption and magnetic circular dichroism (M.C.D.) spectrum of the IrBr6 2- ion at room and liquid helium temperature has been studied in the host crystals (NH4)2SnBr6, K2SnBr6 and (C2H5NH3)2SnBr6 in the region ∼11 000–21 000 cm-1. An interpretation of the spectrum is presented which differs significantly from those suggested previously. In order of increasing energy the allowed bands are assigned to the following ligand-to-metal charge-transfer transitions: Eg ″(2 T 2g )→ Uu ′(2 T 1u ) (13–14 000 cm-1), Eg ″(2 T 2g )→ Eu ″(2 T 2u ) (16 800 cm-1), and Eg ″(2 T 2g )→ Uu ′(2 T 2u ) (∼ 18 300 cm-1). Both our absorption and M.C.D. data strongly suggest a Jahn-Teller splitting of the Uu ′(2 T 1u ) state but contradict a previous suggestion of such a splitting of the Uu ′(2 T 2u ) state. Consideration of σ—π mixing in the t 1u (π + σ) molecular orbital suggests that the ∼17 300 cm-1 band arises from the orbitally-forbidden Eg ″(2 T 2g )→ Eu ′(2 T 1u ) transition. Bands in the 11 000–13 000 cm-1 region are assigned to parity-forbidden charge-transfer transitions to states generated by the t 1g (π)→ t 2g excitation. The fine structure seen at liquid helium temperature in K2SnBr6 : Ir4+ both in the 14 500 cm-1 band and overlying the Eg ″→ Uu ′(2 T 2u ) band appears to be associated with parity-forbidden transitions.