Synthesis and Structural Characterization of a New Cyanomanganate(III) Complex, Heptapotassium µ-Oxo-bis[pentacyanomanganate(III)]cyanide

Abstract

The final product resulting from the reaction of KMnO_4 and KCN in saturated aqueous solution is a gold-brown compound K_7[(CN)_5MnOMn(CN)_5]CN, as established by a single-crystal X-ray diffraction study. Full-matrix leastsquares refinement included anisotropic temperature parameters for all atoms and converged with a final R index (on F) of 0.091. The structure contains the oxo bridged [Mn_2O(CN)_(10)]^(6-) ion, which has 2/m crystallographic symmetry with the bridging oxygen atom lying at a center of symmetry. The ion exists in an eclipsed rotomeric configuration. The Mn-O distance is relatively short, 1.723 (4) Å. Crystallographically independent potassium ions are coordinated to the nitrogen ends of cyanide groups in trigonal prismatic, octahedral, and square antiprismatic geometries. Crystal data are as follows: orthorhombic; space group Ibɑm; ɑ = 12.397 (8), b = 12.772 (8), c = 14.618 (7) A (temperature 23°); Z = 4; d_(obsd) = 1.98, d_(calcd) = 1.97 g/cm^3. The synthesis, isolation, and physical characteristics of K_7[Mn_2O(CN)_(10)]CN are reported along with spectral and magnetic data. The principal features in the infrared spectrum are cyanide stretching bands centered around 2090 cm^(-1). The optical absorption spectrum in a KBr pellet consists of a primary band at 370 nm with a prominent shoulder at 410 nm and a weak shoulder at approximately 610 nm. The gold-brown crystals are strongly pleochroic, the crystals being colorless when the E vector is parallel to the long needle axis. This characteristic serves as a convenient means of identification for this substance. At room temperature K_7[Mn_2O(CN)_(10)]CN is diamagnetic.

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