MAGNETOOPTICAL INVESTIGATION OF THE BROAD SPIN-ALLOWED BANDS IN THE METAL-METAL-BONDED COMPLEXES MO2X9(3-) (X = CL, BR)

The broad spin-allowed bands observed between 16 000 and 25 000 cm-1 in Cs3Mo2X9 (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 t2e --> 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 t2e single-ion orbitals. The spectral features are successfully modeled as a function of the Racah B and C parameters, the splitting between the trigonal t2e and e single-ion orbitals, and the metal-metal pi exchange interaction. From the analysis, the metal-metal pi interaction in the bromide complex is shown to be weaker than that of the chloride complex. Spin-polarized, transition-state calculations using the SCF-Xalpha-SW method were carried out on Cs3Mo2Cl9. Although the spin-singlet sigma(Mo2) --> sigma*(Mo2) transition is calculated around 21 000 cm-1 and is electric-dipole allowed, the low intensity (E(max) < 130) in this region indicates that this transition lies to higher energy, above 27 000 cm-1.