Silenes: Connectors between classical alkenes and nonclassical heavy alkenes

Forty years have passed since the first publication of the experimental evidence for formation of a SiC double bonded compound, a silene. Since then, a large number of transient as well as isolable silenes have been studied, both experimentally and theoretically. Herein, we focus on the impact of the substituents on the electronic and geometric structure, reactivity, and other properties of (formally) SiC double bonded compounds. Qualitative quantum chemical models for the bonding are reviewed, and applied to rationalize experimental observations. Silenes can have planar (classical) structures similar to alkenes, or nonplanar (nonclassical) structures similar to the heavier alkene congeners, and their substituents are pivotal in determining which of these structures is adopted. Silene properties, ranging from charge distribution and NMR chemical shifts to reactivities, are strongly connected to the electronic structure of the silene, and thus to its substitution pattern.