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

Abstract

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_5h) structure (²E₁ “ – (e₁ “)³ ground state) for Mo(CN)₇^4-  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 g_Ʇ = 1.973. The g values and a μ_eff = 1.73 BM are consistent with a ²A₁(a₂)²(a₁)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 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 D_5h Mo(CN)₇^4-  occurs at 24,900 cm^-1.