Syntheses, Structures, and Infrared Spectra of the Hexa(cyanido) Complexes of Silicon, Germanium, and Tin

The rare octahedral EC₆ coordination skeleton type is unknown for complexes with coordination centers consisting of group 14 elements. Here, the first examples of such EC₆ species, the hexacoordinate homoleptic cyanido complexes E(CN)₆^2-, E = Si, Ge, Sn, have been synthesized from element halides SiCl₄, GeCl₄ and SnF₄ and isolated as salts with PPN counterions (PPN⁺ = (Ph₃P)₂N⁺) on a scale of 0.2-1 g. Characterization by spectroscopic techniques and by structure determination through single crystal crystallographic methods show that these pseudohalogen complexes have effective octahedral symmetry in solution and in the solid state. Infrared spectra obtained in solution reveal that the T_{1 u} symmetric IR-active vibrations in all three complexes have unusually small oscillator strengths. The observed reluctance of Si(CN)₆^2-, Ge(CN)₆^2-, and Sn(CN)₆^2- to form from chloro-precursors was rationalized in terms of Gibbs free energies, which were found by ab initio calculations at the CCSD(T)-F12b/aug-cc-pVTZ(-PP)-F12 level of theory to be small or even positive. The work demonstrates that E(CN)₆^2- complexes of silicon, germanium and tin are in fact stable at room temperature and exist as well-defined units in the presence of noncoordinating counterions. The results add to our understanding of the chemistry of pseudohalogens and structure and bonding.