Neutral Pentacoordinate Halogeno- and Pseudohalogenosilicon(IV) Complexes with an SiSONCX Skeleton (X = F, Cl, Br, I, N, C): Synthesis and Structural Characterization in the Solid State and in Solution

A series of neutral pentacoordinate silicon(IV) complexes with an SiSONCX skeleton (X = Cl, Br, I, N, or C) was synthesized and structurally characterized by multinuclear solution-state and solid-state NMR spectroscopy and single-crystal X-ray diffraction. These compounds contain an identical tridentate dianionic S,N,0 hgand, a monodentate (pseudo)halogeno ligand (F, Cl, Br, I, NCS, N(3), or CN), and a monodentate organyl ligand (methyl, phenyl, 4-(trifluoromethyl)phenyl, or pentafluorophenyl). For most of these compounds, a dynamic equilibrium between the pentacoordinate silicon(IV) complex and two isomeric tetracoordinate silicon species in solution was observed. Most surprisingly, comparison of two series of analogous compounds containing fluor, chloro, bromo, or iodo ligands demonstrated that pentacoordination in these series of silicon(IV) complexes is favored in the rank order I approximate to Br> CI>F; i.e., increasing the softness of the halogen ligand favors pentacoordination.