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

A series of tetracyanoborate salts M[B(CN)<sub>4</sub>] with the singly charged cations of Li<sup>+</sup>, Na<sup>+</sup>, Rb<sup>+</sup>, Cs<sup>+</sup>, [NH<sub>4</sub>]<sup>+</sup>, Tl<sup>+</sup>, and Cu<sup>+</sup> as well as the THF solvate tetracyanoborates Na[B(CN)<sub>4</sub>]·THF and [NH<sub>4</sub>][B(CN)<sub>4</sub>]·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)<sub>4</sub>] salts. Crystallographic data for these compounds are as follows:  Na[B(CN)<sub>4</sub>], cubic, <i>Fd</i>3̄<i>m</i>, <i>a</i> = 11.680(1) Å, <i>Z</i> = 8; Li[B(CN)<sub>4</sub>], cubic, <i>P</i>4̄3<i>m</i>, <i>a</i> = 5.4815(1) Å, <i>Z</i> = 1; Cu[B(CN)<sub>4</sub>], cubic, <i>P</i>4̄3<i>m</i>, <i>a</i> = 5.4314(7) Å, <i>Z</i> = 1; Rb[B(CN)<sub>4</sub>], tetragonal, <i>I</i>4<sub>1</sub>/<i>a</i>, <i>a</i> = 7.1354(2) Å, <i>c</i> = 14.8197(6) Å, <i>Z</i> = 4; Cs[B(CN)<sub>4</sub>], tetragonal, <i>I</i>4<sub>1</sub>/<i>a</i>, <i>a</i> = 7.300(2) Å, <i>c</i> = 15.340(5) Å, <i>Z</i> = 4; [NH<sub>4</sub>][B(CN)<sub>4</sub>], tetragonal, <i>I</i>4<sub>1</sub>/<i>a</i>, <i>a</i> = 7.132(1) Å, <i>c</i> = 14.745(4) Å, <i>Z</i> = 4; Tl[B(CN)<sub>4</sub>], tetragonal, <i>I</i>4<sub>1</sub>/<i>a</i>, <i>a</i> = 7.0655(2) Å, <i>c</i> = 14.6791(4) Å, <i>Z</i> = 4; Na[B(CN)<sub>4</sub>]·THF, orthorhombic, <i>Pnma</i>, <i>a</i> = 13.908(3) Å, <i>b</i> = 9.288(1) Å, <i>c</i> = 8.738(1) Å, <i>Z</i> = 4; [NH<sub>4</sub>][B(CN)<sub>4</sub>]·THF, orthorhombic, <i>Pnma</i>, <i>a</i> = 8.831(1) Å, <i>b</i> = 9.366(2) Å, <i>c</i> = 15.061(3) Å, <i>Z</i> = 4. The cubic Li<sup>+</sup>, Na<sup>+</sup>, and Cu<sup>+</sup> salts crystallize in a structure consisting of two interpenetrating independent tetrahedral networks of M cations and [B(CN)<sub>4</sub>]<sup>-</sup> ions. The compounds with the larger countercations (Rb<sup>+</sup>, Cs<sup>+</sup>, Tl<sup>+</sup>, and [NH<sub>4</sub>]<sup>+</sup>) 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 A<sub>1</sub> and T<sub>2</sub> 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.