Tetracyanomanganate(II) and Its Salts of Divalent First-Row Transition Metal Ions

The first known paramagnetic, tetrahedral cyanide complex, [MnII(CN)4]2-, is formed by the photoinduced
decomposition of [MnIV(CN)6]2- in nonaqueous solutions or by thermal decomposition in the solid state. In
acetonitrile or dichloromethane, photoexcitation into the ligand-to-metal charge transfer band (λmax = 25 700
cm-1, ε = 3700 cm-1 M-1) causes the homolytic cleavage of cyanide radicals and reduction of MnIV. Free cyanide
in dichloromethane leads to the isolation of polycyanide oligomers such as [C12N12]2- and [C4N4]-, which was
crystallographically characterized as the PPN+ salt C40H30N5P2:  monoclinic space group = I2/a, a = 18.6314(2)
Å, b = 9.1926(1) Å, c = 20.8006(1), β =106.176(2)°, Z = 4]. In the solid state MnIV−CN bond homolysis is
thermally activated above 122 °C, according to differential scanning calorimetry measurements, leading to the
reductive elimination of cyanogen. The [MnII(CN)4]2- ion has a dynamic solution behavior, as evidenced by its
concentration-dependent electronic and electron paramagnetic spectra, that can be attributed to aggregation of the
coordinatively and electronically unsaturated (four-coordinate, 13-electron) metal center. Due to dynamics and
lability of [MnII(CN)4]2- in solution, its reaction with divalent first-row transition metal cations leads to the
formation of lattice compounds with both tetrahedral and square planar local coordination geometries of the
metal ions and multiple structural and cyano-linkage isomers. α-MnII[MnII(CN)4] has an interpenetrating sphalerite-
or diamond-like network structure with a unit cell parameter of a = 6.123 Å (P4̄3m space group) while a β-phase
of this material has a noninterpenetrating disordered lattice containing tetrahedral [MnII(CN)4]2-. Linkage
isomerization or cyanide abstraction during formation results in α-MnII[CoII(CN)4] and MnII[NiII(CN)4] lattice
compounds, both containing square planar tetracyanometalate centers. α-MnII[CoII(CN)4] is irreversibly transformed
to its β-phase in the solid state by heating to 135 °C, which causes a geometric isomerization of [CoII(CN)4]2-
from square planar (νCN = 2114 cm-1, S = 1/2) to tetrahedral (νCN = 2158 cm-1, S = 3/2) as evidenced by
infrared and magnetic susceptibility measurements. MnII[NiII(CN)4] is the only phase formed with NiII due to the
high thermodynamic stability of square planar [NiII(CN)4]2-.