A variety of transition-metal complexes with terminal silylene ligands have become available in recent years, because of the discovery of several preparative methods. In particular, three general synthetic routes to these complexes have emerged, on the basis of anionic group abstraction, coordination of a free silylene, and alpha-hydrogen migration. The direct transformation of organosilanes to silylene ligands at a metal center (silylene extrusion) has also been observed, and this has further spurred the exploration of silylenes as ligands. This Account describes the synthetic development of silylene ligands in our laboratory and resulting investigations of stoichiometric and catalytic chemistry for these species.

A review of σ-bond metathesis is presented using Watson's 1983 observation of degenerative methyl ligand exchange at metallocene compounds as a starting point. This review has two major parts. The first presents identifying features and reactivity characteristics of this pivotal mechanistic step for high oxidation state metals, which are given with some historical context, though not in historical order for clarity of discussion. The second part presents several selected examples of the exploits of this transformation with a particular focus on catalytic bond forming reactions.

New zirconium triamidoamine complexes (N3N)ZrR (N3N = N(CH2CH2NSiMe3)3 3-; R = Me, 1; PHPh, 2; PHCy, 4) are effective catalysts for the dehydrocoupling of primary and secondary phosphines and select for the diphosphine product. Mechanistic analysis revealed that metal-catalyzed P−P bond formation occurs via σ-bond metathesis steps.

The reactions of nickel complexes bearing terminal imido, phosphinidene, and carbene ligands with ethylene are reported. In all three cases, corresponding three-membered rings, aziridine, phosphirane, and cyclopropane, were produced in moderate to excellent yields. NMR spectra of the phosphinidene (dtbpe)Ni=P(dmp) reaction with ethylene show a [2+2] cycloaddition adduct before phosphirane formation. A labeling study with trans-ethylene-d2 shows formation of aziridine and phosphirane proceeds with net retention of relative stereochemistry.

1-Adamantyl- and mesitylazide react with [(dtbpe)Ni]2(eta2-mu-C6H6) to give the eta2 organic azide adducts (dtbpe)Ni(eta2-N3R) (R = Ad, 3a; Mes, 3b) that have been isolated in good yields and crystallographically characterized. These azide adducts are intermediates in the formation of the corresponding terminal imido complexes (dtbpe)NiNR (R = Ad, 4a; Mes, 4b), undergoing intramolecular loss of dinitrogen upon mild thermolysis.