Magnetooptical investigation of the broad spin-allowed bands in the metal-metal-bonded complexes nonahalodimolybdate(3-), Mo2X93- (X = Cl, Br)

Abstract

The broad spin-allowed bands observed between 16 000 and 25 000 cm in CsMoX (X = Cl, Br) have been investigated by low-temperature single-crystal absorption and MCD spectroscopy. Using the exchange-coupled pair model, it is shown that these bands can be assigned to singly-excited pair states which involve trigonal t → e single-ion excitation. Weaker, sharper features are also observed which are attributed to doubly-excited pair states arising from intraconfigurational transitions (spin-flips) within the t single-ion orbitals. The spectral features are successfully modeled as a function of the Racah B and C parameters, the splitting between the trigonal t and e single-ion orbitals, and the metal-metal π exchange interaction. From the analysis, the metal-metal π interaction in the bromide complex is shown to be weaker than that of the chloride complex. Spin-polarized, transition-state calculations using the SCF-Xα-SW method were carried out on CsMoCl. Although the spin-singlet σ(Mo) → σ*(Mo) transition is calculated around 21 000 cm and is electric-dipole allowed, the low intensity (ϵ < 130) in this region indicates that this transition lies to higher energy, above 27 000 cm.