Magnetism, Bonding and Structure of Coordination Compounds

INTRODUCTION It is not only timely, for the reasons discussed below, to provide a survey of the use of magnetic susceptibility measurements for the study :of complex compounds, but also fitting that the subject should be discussed in a country whose scientists have made so many outstanding contributions to quantum mechanics in general and spectroscopy and magnetism in particular. I pay a special tribute to Professor M. Kotani of the University of Tokyo whose important paper' published in 1949 added much to the classic book by Professor J. H. van Vleck2 on magnetic and electric susceptibilities. It is also noteworthy that 1967 marks the centenary of the death of Michael Faraday who discovered many effects relating magnetism and structure, quite apart from his work on electro-magnetism. A decade ago there was widespread optimism concerning the increasing value of magnetic susceptibility measurements for the study of chemical structure. In more recent years, as the effects of structural distortions, electron delocalization, ion—ion interactions etc. on magnetic moments have been shown to be more serious than was initially believed to be the case, considerable reservations concerning the usefulness of magnetic measurements have been expressed3. Here we propose to try to outline the value and limitations of magnetic measurements for the chemist and will conclude that even though for some purposes, e.g. in connection with stereochemistry, magnetism is less useful than once thought, it still provides a very valuable fechnique for studying many other aspects of structure. We survey the subject under three main headings: 1. A brief outline of theoretical principles with special reference to the magnetic behaviour of d8 metal complexes—a configuration chosen because of the versatility of oxidation states, coordination numbers, stereochemistries and bond types displayed by many different elements. 2. A discussion of the various kinds of structural information about metal complexes which can be inferred from magnetic measurements. Special attention is drawn to the reliability and the limitations in each case. 3. An indication of the areas where magnetic studies are developing and where more work is needed to enable magnetic measurements to be reliably interpreted. We are concerned mainly with magnetic susceptibility measurements, supplemented, where useful, by spectroscopic, electron spin resonance and nuclear magnetic resonance studies. The term "structure" is widely inter-