Octahedral monomeric molybdenum(III)

Abstract

The first magneto-optical study of the ⁴ A _2g → ² T _2g absorption, ² E _g → ⁴ A _2g and ² T _2g → ² T _1g, ² E _g luminescences of MoCl^3- ₆ in Cs₂NaYCl₆, as well as the absorption spectrum of the spin-allowed ⁴ T _2g and ⁴ T _1g ligand field states, are reported. Magneto-optical methods have allowed difficulties in a previous assignment to be resolved. Coupling of almost all electronic origins to α_1g, t _2u and both t _1u MoCl^3- ₆ internal modes, as well as t _1u lattice modes, is established, uniquely, from the sign of the magnetic circular dichroism and magnetic circular polarization of luminescence spectra. In addition, coupling to a t _1u internal mode of the host YCl^3- ₆ unit is observed. Moderate coupling to the e _g Jahn-Teller mode occurs for the ² T _2g(U′), ²T_1g(U′) and ² E _g(U′) origins, particularly the former, but is very weak or absent for the ² T _2g(E″) and ² T _1g(E′) origins. A comparison of the relative dipole strengths induced by t _1u and t _2u vibrations, as well as the sign of the MCPL for the ² E _g → ⁴ A _2g transition, are consistent with the intensity for the ⁴ A _2g → ² E _g, ² T _1g, ² T _2g transitions being borrowed primarily from the ⁴ T _2g state via spin-orbit coupling. The observed ² T _1g and ² T _2g spin-orbit splittings of 102 and 75cm^-1, respectively, are reduced by approximately one third from those calculated, and this reduction is attributed to a Jahn-Teller effect.