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Tetracyanoborate Salts M[B(CN)4] with M = Singly Charged Cations:  Properties and Structures

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posted on 21.02.2005 by Torsten Küppers, Eduard Bernhardt, Helge Willner, Henning W. Rohm, Martin Köckerling
A series of tetracyanoborate salts M[B(CN)4] with the singly charged cations of Li+, Na+, Rb+, Cs+, [NH4]+, Tl+, and Cu+ as well as the THF solvate tetracyanoborates Na[B(CN)4]·THF and [NH4][B(CN)4]·THF were synthesized and their X-ray structures, vibrational spectra, solubilities in water, and thermal stabilities determined and compared with already known M[B(CN)4] salts. Crystallographic data for these compounds are as follows:  Na[B(CN)4], cubic, Fdm, a = 11.680(1) Å, Z = 8; Li[B(CN)4], cubic, P4̄3m, a = 5.4815(1) Å, Z = 1; Cu[B(CN)4], cubic, P4̄3m, a = 5.4314(7) Å, Z = 1; Rb[B(CN)4], tetragonal, I41/a, a = 7.1354(2) Å, c = 14.8197(6) Å, Z = 4; Cs[B(CN)4], tetragonal, I41/a, a = 7.300(2) Å, c = 15.340(5) Å, Z = 4; [NH4][B(CN)4], tetragonal, I41/a, a = 7.132(1) Å, c = 14.745(4) Å, Z = 4; Tl[B(CN)4], tetragonal, I41/a, a = 7.0655(2) Å, c = 14.6791(4) Å, Z = 4; Na[B(CN)4]·THF, orthorhombic, Pnma, a = 13.908(3) Å, b = 9.288(1) Å, c = 8.738(1) Å, Z = 4; [NH4][B(CN)4]·THF, orthorhombic, Pnma, a = 8.831(1) Å, b = 9.366(2) Å, c = 15.061(3) Å, Z = 4. The cubic Li+, Na+, and Cu+ salts crystallize in a structure consisting of two interpenetrating independent tetrahedral networks of M cations and [B(CN)4]- ions. The compounds with the larger countercations (Rb+, Cs+, Tl+, and [NH4]+) crystallize as tetragonal, also with a network arrangement. The sodium and ammonium salts with the cocrystallized THF molecules are both orthorhombic but are not isostructural. In the vibrational spectra the two CN stretching modes A1 and T2 coincide in general and the band positions are a measure for the strength of the interionic interaction. An interesting feature in the Raman spectrum of the copper salt is the first appearance of two CN stretching modes.