Unusual silicon coordinations occur fi.as transition structures or reactive intermediates in the process of substituent exchange and in condensation reactions.
Most of the unusually coordinated silicon compounds are highly reactive and consequently difficult to synthesize. Ab initio calculations allow to compute reliable data, such as thermodynamic stabilities or characteristic Si NMR chemical shifts.
The research on low coordinated compounds focuses on silylenes (coordination number 2), silenes, and silyl anions (coordination number 3). For higher coordinated (coordination number 5, 6) silicon molecules apicophilicity rules are investigated, and the effect of inter- and intramolecular coordination of donor groups (fi. amino- or phosphino groups).