Several salts containing the Mo(CN)_7^(4-) ion have been studied by a variety of spectroscopic and magnetic techniques. Infrared and Raman spectral data together with magnetic susceptibility measurements suggest a pentagonal-bipyramidal (D_(Sh)) structure (2E_1"-(e_1")^3 ground state) for Mo(CN)_7^(4-) in aqueous solutions of K_4Mo(CN)_7•2H_2O. Anhydrous K_4Mo(CN), and solid Cs_4Mo(CN)_7xH_2O also appear to contain pentagonal-bipyramidal anions. Solid K_4Mo(CN)_7•2H_2O, however, exhibits infrared and Raman spectra that differ from the aqueous solution results. The esr spectrum of the dihydrate, which can be observed only below 150ºK, gives = 2.103 and g_⊥= 1.973. The g values and a µ_(eff) = 1.73 BM are consistent with a ^2A_1(a_2)^2(a_1)^1 ground state of a monocapped trigonal-prismatic (C_(2v) coordination geometry. Electronic spectral bands at 16,100, 20,900, and 22,500 cm^(-1) in crystalline K_4Mo(CN)_7•2H_2O are assigned respectively to the d-d transitions ^2A_1 → ^2A_2, ^2A_1, → ^2B_2, and ^2A_1, → ^2B_1,. The lowest spin-allowed d-d transition in D_(sh) Mo(CN)_7^(4-), occurs at 24,900 cm^(-1).
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Rossman et al. (1973) studied this question.