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.

Several diorganoscandium complexes stabilized by the β-diketiminato ligands (Ar)NC(R)-CHC(R)N(Ar) (Ar ) 2,6-iPr-C 6 H 3 ; R ) CH 3 (ligand a), R ) tBu (ligand b)) have been synthesized. Reaction of the lithium salts of the ligands with ScCl 3 ‚3THF leads to the complexes LScCl 2 (THF) n , which may be readily alkylated to form the dialkyl derivatives. Most are isolated as base-free, four-coordinate complexes. Several have been characterized via X-ray crystallography, and a detailed discussion of their structures is presented. Steric interactions between Ar and the Sc-alkyl groups force the scandium to adopt an out-ofplane bonding mode. In solution, this is manifested via a fluxional process which equilibrates the two diastereotopic alkyl groups and ligand groups as well. The barriers to this process roughly correlate with the steric bulk of the alkyl substituents. At elevated temperatures, the dialkyl derivatives LScR 2 undergo a metalation process whereby one of the alkyl groups is eliminated as RH, and a ligand iPr group is metalated in the methyl position. These reactions are first order in scandium complex, and activation parameters of ∆H q ) 19.7(6) kcal mol -1 and ∆S q ) -17(2) cal mol -1 K -1 were measured for the loss of Me 4 Si from (Ligb)-Sc(CH 2 SiMe 3 ) 2 .

Interest in the utility of polylactide as a commodity polymer has increased significantly in recent years due to numerous environmental advantages over conventional petrochemically derived plastics. As such, the development of novel catalyst systems for the ring opening polymerization of lactide has seen tremendous progress in the past decade. In particular, divalent metals (i.e. Mg, Ca and Zn) supported by monoanionic ancillary scaffolds are appealing because of their low toxicity and cost. A much less common approach involves the use of neutral ligands in combination with the aforementioned divalent metal centres. The additional valence thus renders it possible, upon reaction with traditional Lewis or Brønsted acid activators, to generate sterically and electronically unsaturated species, akin to the most widely employed olefin polymerization catalysts. This Perspective is not intended as a comprehensive review, but rather a systematic highlight of key contributions, which have served to extend the forefront of this exciting field.

Addition of bulky primary silanes to the osmium benzyl compound, Cp*(iPr3P)OsCH2Ph, afforded two neutral hydrogen-substituted silylene complexes via activation of two Si-H bonds. These species have been structurally characterized, and their reactivity has been examined experimentally and computationally. Comparison of these neutral silylene complexes with their cationic analogue highlights the dramatic influence of charge distribution on the reaction chemistry of metal silylene complexes.

A bulky anilido-imine donor that marries the attributes of the β-diketiminato and salicylaldiminato ligand frameworks has been prepared and used to stabilize bis-alkyl yttrium derivatives, which act as precursors to cationic organoyttrium complexes.

Cyclometalative C-H bond activation is a process that is commonly encountered in the field of organometallic chemistry. In rare earth and actinide complexes, ligand cyclometalation is most prevalent in highly reactive alkyl and hydrido species. Numerous factors promote ligand cyclometalation and influence the rate at which it occurs. This tutorial review discusses key issues relevant to ligand cyclometalation in rare earth and actinide complexes, including kinetic and mechanistic considerations. A variety of examples is presented for a wide range of ligand types and metals, the scope of which is intended to include routine cases, while also highlighting exceptional cyclometalation reactions that lead to unusual bonding modes. The reaction chemistry of cyclometalated rare earth and actinide complexes with various small molecule substrates (e.g. phenol, anilines, triethylammonium salts, alkynes, olefins, hydrogen and hydrocarbons) is also outlined.

The base-free dimethyl scandium complex supported by the bulky beta-diketiminato ligand ArNC((t)Bu)CHC((t)Bu)NAr (Ar = 2,6-(i)Pr(2)C(6)H(3), 1) reacts with various equivalencies of the strong organometallic Lewis acid B(C(6)F(5))(3) to give scandium alkyl cations. With 0.5 equiv, a monocationic mu-methyl dimer (2) was observed spectroscopically. Reaction with a further 0.5 equiv of borane gives the monomeric methyl cation 3, which was fully characterized, including via X-ray crystallography. This compound is fluxional on the NMR time scale via a "ligand flip" mechanism. Reaction with another equivalent of borane gives the unique dication 4, which exhibits a static structure on the NMR time scale. Dimethyl compound 1 is a highly active catalyst precursor for ethylene polymerization under borane or MAO-type activation. Activities for this group 3 metal based catalyst approach those observed for group 4 based metallocene systems.

An osmium complex bearing a terminal hydrogen-substituted stannylene ligand, Cp*((i)Pr(3)P)(H)Os=SnH(trip) (1) (trip = 2,4,6-triisopropylphenyl), has been prepared by stannylene extrusion, and the complex has been structurally characterized. Complex 1 coordinates Lewis bases and activates the O-H bonds of water and methanol. Most interestingly, 1 converts to the metallostannylene complex Cp*((i)Pr(3)P)(H)(2)OsSn(trip) (2) thermally or photochemically by what appears to be a radical process.