A study of the aliphatic Schiff's base complexes of some transition metals

Abstract

It may be said that the study of coordination compounds began with the discovery of the cobalt ammines in the last few years of the eighteenth century. During the next ninety years several theories were advanced to explain the formation of these compounds. However, it was Werner's ideas on coordination complexes (advanced in 1891) which laid the foundation for modern developments in the study of coordination compounds. The two modern theories which are now used (with modifications) to explain the behaviour of these compounds were developed in the 1930's. They are Pauling's valency bond theory and the electrostatic theory, which, combined with the molecular orbital treatment, has become the ligand-field theory. Pauling Pauling: J.A.C.S., 53, 1367 (1931); J.A.C.S. 54, 994 (1932) advanced his theory to explain the fact that many coordination compounds of transition metals exist in two forms which have different magnetic moments. He suggested that there are two fundamentally different types of coordination compound. The "ionic" salts, which had the same magnetic moments as the "free" ions, were assumed to have bonds which were essentially of an electrostatic nature. The other type of complex had "covalent" bonds, (n-l)d orbitals being involved in the bonding. To make (n-l)d orbitals available for hybrid bond formation some of the (n-l)d electrons might be paired, causing a reduction in the value of the magnetic moment from that of the free ion.