LAUSR

Abstract This review collects the examples of a rare, six-coordinate geometry, the capped trigonal bipyramid (CTB). Examples have been identified for d 0 Sc(III), Y(III), Zr(IV), Ta(V), d 4 Mo(II), d 2 Mo(IV) and W(IV), d 2 Re(V), and d 10 Zn(II) and Cd(II). These distorted geometries are caused by bulky thiolate groups or tripodal tetradentate triamidoamine or tetraamine ligands that impose a pseudo C 3 symmetry with two capping atoms that form a distorted Y shape with the metal and the axial donor ligand. The important angles of the structures are highlighted to illustrate this. The environment of the capping atoms is crowded and so the ligands that contribute these atoms have a small steric profile. These include hydrides, small alkyls, alkylidenes, alkyne, phosphines, azobenzene, CO and tetrahydrofuran and the bidentate anions benzamidinate, carbonate, hydrazide and nitrate. The metals that accommodate this crowding tend to be the larger 4d and 5d metals. Several of the complexes are hydrides with a small HML angle at the top of the Y-shape ranging from 50° to 80°. This results, in the case of phosphine complexes, in a large J PH NMR coupling constant of 50–100 Hz between the cis hydride and phosphorus nuclei. An alkylidene–alkyl adduct from diphenylacetylene also has the CTB geometry as well as, possibly, a cationic lutidine adduct with an agostic methyl C H…Mo bond. Zinc(II) and three cadmium(II) complexes with tripodal nitrogen-donor ligands and capping bidentate anions also have the CTB geometry. Several types of reactions have been observed at the capping position including Wittig-like reactions, protonation, reactions with dihydrogen, and reactions involving alpha and beta hydride elimination and addition. The observed reactions related to carbon dioxide fixation, dinitrogen reduction and hydrodesulfurization make such complexes worthy of further examination directed toward catalyst development.