Six coordinate capped trigonal bipyramidal complexes

This is an accepted manuscript published by ElsevierThis review collects the examples of a rare, six-coordinate geometry, the capped trigonal bipyramid (CTB) from the work of the groups of Richards, Schrock and Morris. Examples have been identified for d0 Sc(III), Y(III), Zr(IV), Ta(V), d4 Mo(II), d2 Mo(IV) and W(IV), d2 Re(V), and d10 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 C3 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 and the metals that accommodate this crowding tend to be the larger 4d and 5d metals. Many 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 JPH NMR coupling constant of 50 to 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. A feature common to several structures is that the atoms at the top of the Y are in the plane of one of the equatorial metal-ligand bonds. 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.RHM thanks NSERC for a Discovery Grant and the Canada Council for the Arts for a Killam Research Fellowshi