I C. J. Ballhausen University of Copenhagen Copenhagen. Denmark
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Quantum Mechanics and Chemical Bonding in Inorganic Complexes I. Static concepts of bonding; d y ~ m i concepts c of valency
Today we realize that the whole o f chemistry i s one huge manifestation of quantum phenomena.. . Let us therefore thinkin deep gratitude and admiration of those pioneers who opened the doors to modern chemistry. They are thegiants on whose shoulders theoretical chemistry is standing.
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The Static Concepts ol Chemical Bonding It is a fact that chemical entities exist which are made un. of atoms and which have a well-defined composition and steric arrangement; the atoms are "bonded" together t o form molecules. "Why?" and "How?" were questions asked by chemists ever since Dalton formulated his empirical laws. Around the turn of the nineteenth century the chemists were well acquainted with the rules mvernina the formation of molecules and ions from their con&ituents~and could talk about polar or non-polar linkages. T o this came the coordinate links of Werner. Valency is, of course, just a general term used t o describe the power which atoms have of combining with each other t o form molecules. We know now t h a t chemical bonding parameters soan a wide ranee. from several electron voltslmole found in ;he so-called "&dent2' bonds through k i l o c a b rieslmole for "hvdroaen - .. bonds" and the fraction of kilocalories/mole observed in the "Rydberg molecules." Here I shall, however, onlv he concerned with molecular bonding in the electron volt class. Quite early after the discovery of the electron by J. J. Thomson in 1897 many scientists suggested t h a t molecules might he held together by the electrostatic force resultingfrom a transfer of an electron from one atom t o another. A ereat advance was made by G. N. Lewis (1 ),who in 1916 suggested t h a t it was possible for a n electron to he shared between two atoms, thereby giving a stability t o both. A divalent atom like oxvaen needs. for instance. two more valencv electrons to fill anoctet of elekrons. It cangain these by sharing two electrons ~
belonging t o another atom or atoms, thereby filling both their octets of electrons a t the same time. Lewis did not discuss how this sharing took place, and of course his static model of the chemical honding would require some unknown forces. Sixteen years later he wrote (2) . . . when I first deduced the ides of the electron pair hand from an analysis of chemical facts. . .it was obviously incompatible with the then accepted laws of electromagneticsand mechanics. The qualitative orincinles of molecular structure were oresented. so tosneak. as About the Author.. Carl J. Ballhausen, Professor of Chemistry in the Institute for Physical Chemistry at the University of Copenhagen, has earned an international reputation through his scientific research, scholarly writings, and concerns for chemical education. His article on quantum mechanics and chemical bonding, the first part of which appears here, provides a unique and fitting introduction to the man and his work. This is the first part of Carl Rallhausen's eontrihutiun to our understanding of the "influence and development of quantum mechanical ideas as applied to inorganic complexes." The second Dart initiates a discussion of valencv and inoreanic metal comappear in the June issue.
Eric S. Proskauer (right) received his doctoral degree in physical chemistry from the University of Leipzig. His early work as a writer, editor, and student of the literature of chemistry led him into publishing and a life-long relationship with the creation of scientificliterature that has already spanned 50 years. He was eo-founder of Interscience Publishers and, eventually, director and senior vice president of the Wiley-Interscience Division of John Wiley and Sons. In semi-retirement now, his contributions to the literature of chemistry continue. Leonard W. Fine (left) received his doctoral training in organic chemistry at the University of Maryland. He is currentlyProfessor of Chemistry at Housatonie College (State of Connecticut) and lecturer in the General Studies Program at Columbia University. His three textbooks and active interest in the work of the Division of Chemical Education suezest a erowine concern amone vouneer
Volume 56, Number 4. April 1979 / 215
A great advance was made by G. N. Lewis, who in 1916 suggested that it was possible for an electron to be shared between two atoms
the minimum demands of the chemist which must eventually be met by the more far-reaching and quantitative work of the mathematical physicist. ' No understanding of the mechanisms and dynamics of a chemical bond was indeed possible before the advent of Schrodinaer's wave eauation in 1926. Yet it should not he forgottenthat in a qualitative way the Lewis concept of the chemical bonds offers a remarkably successful picture of many valence phenomena. Its impact on the understanding and teaching of chemistry can hardly he exaggerated.
The Dynamic Conce~tsof Valency
The year IPX cclearlv marks the outset o f a new era in rheorerical physics and chrmistry. We shall Dausr at this moment in the histbry of wave mechanics and chemical bonding and take stock of the situation. In the preface to his hook "The Electronic Theory of Valency" published in 1927 N. V. Sidgwick writes: It has been suggested that the development in the last few years of the theory of wave mechanics necessitates a fundamentalchange in our views of atomic structure. This theory, in the hands of de Broglie, Heisenberg, SchrBdinger, and others, has had the most remarkable success in dealing with problems of atomic physics . . . [but] the electron as a separate entity seems to disappear from physics.. . .The theory of wave mechanics, although there can be no doubt of its value as a calculus, has not yet reached the stage at which one can say definitely how it is t o be translated into physics. . ..It bas as yet given no "roof that the ohvsical conceots which led Schrbdineer to his , fundamrnrnldiiferential rquatmn shzmld hr taken s u lirernlly as to he inrompotihlr with the ion
