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

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, Fd3̄m, 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.