Rossman, F.-D. Tsay, Harry B. Gray
Arthur Noyes Laboratory of Chemical Physics
California Institute of Technology, Pasadena, CA 91109
Several salts containing the Mo(CN)74- 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 (D5h) structure (2E1 “ – (e1 “)3 ground state) for Mo(CN)74- in aqueous solutions of K4Mo(CN)7·2H2O. Anhydrous K4Mo(CN)7 and solid Cs4Mo(CN)7·xH2O also appear to contain pentagonal-bipyramidal anions. Solid K4Mo(CN)7·2H2O, 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 150°K, gives g|| = 2.103 and gꞱ = 1.973. The g values and a μeff = 1.73 BM are consistent with a 2A1(a2)2(a1)1 ground state of a monocapped trigonal-prismatic (C2v) coordination geometry. Electronic spectral bands at 16,100, 20,900, and 22,500 cm-1 in crystalline K4Mo(CN)7·2H2O are assigned respectively to the d-d transitions 2A1 -> 2A2, 2A1 -> 2B2, and 2A1 -> 2B1. The lowest spin-allowed d-d transition in D5h Mo(CN)74- occurs at 24,900 cm-1.