Spectroscopic and Magnetic Properties of Heptacyanomolybdate(III).
Evidence for Pentagonal-Bipyramidal and Monocapped Trigonal-Prismatic Structures

George R. 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)72H2O. Anhydrous K4Mo(CN)7 and solid Cs4Mo(CN)7xH2O also appear to contain pentagonal-bipyramidal anions. Solid K4Mo(CN)72H2O, 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 150K, gives g|| = 2.103 and g = 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)72H2O 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.

last updated: 28-Jul-2023