Vibrational properties and bonding analysis of copper hexacyanoferrate complexes in solid state

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Vibrational spectroscopic study of crystalline copper hexacyanoferrate complexes of composition K4CuII6 [FeII(CN)6]4·nH2O (1) and CuII6[FeIII(CN)6]4·nH2O (2) with -Cu-N≡C-Fe- bridging structures have been performed. The cubic Fm3¯m (Oh5) unit-cells contain ideally 4 Fe and 4 Cu ions which were calculated by periodic density functional theory (DFT) (using the Gaussian09 C.01 software package) for ideal lattice compositions of K8CuII4[FeII(CN)6]4 (1a), K4CuII4[FeIII(CN)6]4 (2a) and with lattice water molecules KCuII4[FeIII(CN)6]3·6H2O (3a). Systematically, non-linear Cu-N≡C structure was fitted with Cu-N≡C bond angles about 155° for complexes 1a, 2a, and 3a. Practically, all optically active internal modes of Fe(CN6)n− moieties resulted from factor group analysis as 4A1g + 6Eg + 4F1g + 10F1u were experimentally observed and assigned. Some low-frequency translatory and librational modes were also interpreted. Vibrational bands were assigned to cis- and trans-Cu(NC)4(OH2) complexes which are formed in the lattice holes of both complexes. Vibrational spectra and force constants of a great number of transition metal hexacyano complexes of compositions K4[MII(CN)6], K3[MIII(CN)6], CsLi2[MIII(CN)6] and Prussian blue analogues have been reexamined and recalculated. Internal and external modes of 6 different lattice water species (coordinated, hydrogen bonded, or zeolitic type) have been interpreted for complex 2 using results of periodic DFT calculation of model complex 3a.