Experimental charge density studies of cyclotetrasilazane and metal complexes containing the Di- and tetraanion

The homologous series of parent octamethylcyclotetrasilazane (c-NH-SiMe₂-)₄, (1), the lithium complex [(THF) ₂Li₂-(c-N-SiMe₂-NH-SiMe₂-) ₂]₂, (2), containing the cyclic dianion, and [(THF) ₂LiAl(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.