Experimental charge density studies of cyclotetrasilazane and metal complexes containing the di- and tetraanion - Research report

The homologous series of parent octamethylcyclotetra-silazane (c-NH-SiMe2-)(4), (1), the lithium complex [(THF)(2)Li-2-(c-N-SiMe2-NH-SiMe2-)(2)](2), (2), containing the cyclic dianion, and [(THF)(2)LiAl(c-N-SiMe2-)(4)](2), (3), accommodating the unprecedented tetraanion [Me2SiN](4-) was synthesized to investigate the nature of the covalent Si-N single bond in the presence of various metals. These model compounds show a wide diversity of Si-N(H), Si-N(M), Si-N(H,M) and M-N bonds and serve as bench-mark systems to study polar bonds by high-resolution low-temperature X-ray structure analysis. Experimental charge density studies reveal highly polar Si-N bonds with remarkable ionic contribution,even in the non-metallated starting material 1. The Li-N and Li-O bonds have to be classified as almost purely ionic bonds with topological properties not far from those determined for NaCl.