Metal‐Olefin and ‐Acetylene Bonding in Complexes

The molecular orbital (MO) and valence bond (VB) descriptions of the bonding in metalolefin and ‐acetylene complexes are examined in relation to their ability to explain the known properties of these complexes. It is concluded that whereas there are difficulties with the valence bond description, the molecular orbital description can satisfactorily explain all the known properties of these complexes and has the necessary versatility to enable it to be readily applied to any series of metal‐olefin or ‐acetylene complexes. It is further shown that olefin and acetylene complexes can be usefully divided into two main classes: those that resemble the platinum( II )‐complexes (class S), and are either square‐planar or octahedral with essentially free rotation about the metal‐olefin bond; and those that resemble platinum(0)‐complexes (class T) and are either trigonal planar or trigonal bipyramidal and in which there is no rotation about the metal‐olefin or ‐acetylene bond.