1 M e t a l - M e t a l Multiple Bonds and M e t a l Clusters New Dimensions and New Opportunities in Transition Metal Chemistry
Downloaded by 41.138.93.166 on December 25, 2015 | http://pubs.acs.org Publication Date: June 11, 1981 | doi: 10.1021/bk-1981-0155.ch001
F. A. COTTON Department of Chemistry, Texas A&M University, College Station, TX 77843
The chemistry of the transition metals became a mature field, based on a sound foundation of broad, basic principles around the turn of the century as a result of the efforts of Alfred Werner, who was the first chemist to make sense out of an enormous body of experimental facts that had been painstakingly accumulated by himself, by the Danish chemist S. M. Jørgensen, and by a number of others. Working without the aid of any direct structural data, Werner was able to combine intuition, geometric reasoning and experiment to develop the central principle of his coordination theory, namely, that an ionized transition metal atom would always be surrounded by a set of neutral molecules (e.g., H O, NH ) and/or anions (e.g., CN , Cl , OH ) arranged in a geometrically well-defined pattern (square, tetrahedral, octahedral). This general conception dominated transition metal chemistry for more than six decades and even today is applicable to a vast amount of chemistry, albeit with the benefit of some further knowledge and insight, such as: (1) recognition that coordination numbers other than those emphasized by Werner (e.g., five-, seven- and eight-coordination) are important; (2) recognition that coordination geometry, especially for 5-, 7- and 8-coordination, is not always rigid; (3) much more detailed knowledge of equilibria, kinetics and spectra; (4) a quantum-mechanically sound understanding of metal-ligand bonding. -
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There are, however, s e v e r a l aspects of contemporary t r a n s i t i o n metal chemistry whose e x i s t e n c e could not have been extrapolated from the Wernerian p r i n c i p l e s . Among these one could mention considerable areas of metal carbonyl type chemist r y , much of the current field of organometallic chemistry and, most unambiguously, the chemistry of compounds c o n t a i n i n g metal-metal bonds. Although Werner d e a l t e x t e n s i v e l y with p o l y n u c l e a r complexes, these were conceived simply as two or more mononuclear complexes u n i t e d only by the ligands they shared. 0097-615 6 / 81 / 0 1 5 5-0001 $ 0 5 . 0 0 / 0 © 1981 American Chemical Society
In Reactivity of Metal-Metal Bonds; Chisholm, Malcolm H.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.
REACTIVITY
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M E T A L - M E T A L
BONDS
The n o t i o n of d i r e c t metal-metal bonds i s , to the best of my knowledge, t o t a l l y absent from the work of Werner, h i s coworkers or h i s d i r e c t f o l l o w e r s . I t i s a modern, "non-Wernerian" concept, and i t i s the subject of t h i s symposium. Even while Werner was s t i l l a l i v e , the f i r s t non-Wernerian compounds were discovered. In 1907 the compound " T a C ^ ^ l ^ O " was reported and i n 1913 reformulated, c o r r e c t l y , as TagCli^·7Η2θ (1) · During the t h i r d decade of t h i s century polynuclear h a l i d e compounds of molybdenum were discovered (2). The true s t r u c t u r e s of these compounds could not be i n f e r r e d by the a v a i l a b l e experimental methods and d e s p i t e t h e i r f a i l u r e to f o l l o w the general (Wernerian) patterns of behavior they a t t r a c t ed l i t t l e a t t e n t i o n . Even when, i n 1935, C. Brosset (3) showed that t h e t u n g s t e n atoms i n [ ^ C l g ] ^ were very c l o s e together (ca. 2.5A) and again a decade l a t e r (4) showed that the a f o r e mentioned lower h a l i d e compounds of molybdenum contained octahedral M 0 5 groups with Mo-Mo distances about equal to the interatomic d i s t a n c e i n m e t a l l i c molybdenum, no new research appeared i n the l i t e r a t u r e . The presence of Tag octahedra i n the lower tantalum c h l o r i d e was discovered i n 1950 and i t was e x p l i c i t l y concluded that metal-metal bonds were present i n t h i s and r e l a t e d compounds (5), but s t i l l , no one apparently was i n t e r e s t e d i n l o o k i n g f u r t h e r at such " c u r i o s i t i e s . " It was i n 1963, with yet another a c c i d e n t a l d i s c o v e r y , that of the [ R e C l ] i o n (6, 7) that the f i e l d of "metal atom c l u s t e r " chemistry r e a l l y had i t s b i r t h , since t h i s discovery provoked the f i r s t general d i s c u s s i o n of the existence and probable importance of the e n t i r e c l a s s of "metal atom c l u s t e r " compounds (6). I t was i n the Re^ c l u s t e r s a l s o that the f i r s t m u l t i p l e M-M bonds were e x p l i c i t l y and unequivocally recognized (6) and t h i s was soon followed by the d i s c o v e r y of the f i r s t quadruple bond (8) and then the f i r s t t r i p l e bond (9). Since these seminal d i s c o v e r i e s , the f i e l d of metal atom c l u s t e r s and M*-M m u l t i p l e bonds has a r i s e n , grown and f l o u r i s h e d .
Downloaded by 41.138.93.166 on December 25, 2015 | http://pubs.acs.org Publication Date: June 11, 1981 | doi: 10.1021/bk-1981-0155.ch001
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Survey of M-M
Bonds and C l u s t e r s
The compounds under c o n s i d e r a t i o n here are r e s t r i c t e d to those i n which there are d i r e c t M
