Iron salts with the tetracyanidoborate anion: [Fe(III)(H2O)6][B(CN)4]3, coordination polymer [Fe(II)(H2O)2{κ2N[B(CN)4]}2], and [Fe(II)(DMF)6][B(CN)4]2.

The reaction of Fe(OH)(3) with tetracyanidoboronic acid, H[B(CN)(4)]·xH(2)O, in water leads to the first examples of tetracyanidoborates with a triply charged metal cation, [Fe(III)(H(2)O)(6)][B(CN)(4)](3) (1). Using elemental iron powder as starting material, [Fe(II)(H(2)O)(2){κ(2)ΝB(CN)(4)]}(2)] (2) is obtained. Anhydrous iron(II) tetracyanidoborate, which is synthesized by heating of 2, is soluble in dry dimethylformamide. After evaporation of the DMF solvent, single crystals of the third title compound, [Fe(II)(DMF)(6)][B(CN)(4)](2) (3), are obtained. Compound 3 is the first metal tetracyanidoborate soluble in nonpolar solvents. The title compounds have been characterized by single-crystal X-ray diffraction (1 rhombohedral, R3c (no. 167), a = 14.9017(7) Å, c = 20.486(1) Å, Z = 6; 2 tetragonal, I42d (no. 122), a = 12.3662(3) Å, c = 9.2066(4) Å, Z = 4; 3 triclinic, P1 (no. 2), a = 8.6255(3) Å, b = 11.0544(4) Å, c = 12.2377(5) Å, Z = 1). The metal ions in all three compounds are octahedrally coordinated. Whereas 1 and 3 are built up from isolated complex ions, 2 comprises a coordination polymer, in which the Fe(II) ion is coordinated by two oxygen atoms of two water molecules in a trans orientation and four nitrogen donor atoms of the [B(CN)(4)](-) groups, which bridge between neighboring iron ions. The iron(III) ion in 3 is in a perfect octahedral environment, which is formed by the O atoms of 6 molecules of water. The single-crystal X-ray structures, vibrational spectra, thermal properties, solubilities, and electrochemical characteristics are reported and compared with those of other known tetracyanidoborates.