Edited by Charles D. Mickey Texas A 8 M at Galveston Galveston. TX 77553
revi~i ted Some Aspects of Coordination Chemistry Charles D. Mickey Texas ABM University at Galveston, Galveston. TX 77553 Chemists have been intrieued hv coordination comnounds for almost two centuries. ~ i e i h a c h , Prussian a pigrnen&aker, svnthesized the first metal coordination compound a t the beginning of the 1Ahcentury.This iron hluc &ment,named I'russ~an hlue, has the composition KFestCNk. The historiral development of coordination chemistry begins, however, with B. M. Tassaert's report (1) in 1798 of the discovery of hexaamminecohalt(II1) chloride. ICo(NHl)clCL. . ".", The unusual characteristics of this A d simil'a; compounds stimulated interest and research in this area of inoreanic chemistrv. Since the properties of such compounds seemed to defy explanation by contemporary chemical theory they were characterized as complex compounds. Today coordination chemistry, the study of coordination compounds, has emerged as one of the major focal points for the inorganic chemist. I t covers a comprehensive range of intriguing theoretical and practical applications. In industry, coordination compounds play key roles in homogeneous and heterogeneous catalysis, water purification, analytical chemistry, solvent extraction, photography, metallurgy, and electrochemistrv. In hiolow. thev are essential in manv life processes such as oxygen transport and metal ion control. Moreover. the studv of coordination comnounds has enabled the inorganic chemist to mnke aignificant progress in refining and amvlifvine the concept of chemical bondine and to explain th'r ir?luence that hhnding has on a comp&d's properties like its color and magnetic s~tsceptihility.
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The Genesis of Modern Coordination Theory By 1852, Sir Edward Frankland in England had proposed the concept of valence (21, suggesting that each element f o m s compounds by uniting with a definite number of other elements. In 1857. Friedrich A. Kekul6 in Germanv extended Frankland's concept, advocating the tetravalenc; of carbon (3).Subsequently, Kekul.6 proposed that carbon atoms comhine to form chains ( 4 ) . The theories extolled by Frankland and Kekul.6 were crucial for developing a structural theory for organic compounds; however, they did influence and hinder the development of coordination theory. Thus, Christian W. Blomstrand (5)proposed that ammonia molecules could form -NH3chains, resembling the -CH2chains in organic com~ounds.Bv extendine this idea. S o ~ h u Mads s Joreensen a chi& theory & account for the existence ormetal comnlexes (6).Bv analogv to organic comnounds, he formulated tetraammin~platin&n(II)chloride asshown in structure
Although Jorgensen's conjectures seem unsophisticated now, they were in accord with the available facts of his time. To his credit, Jorgensen was an extraordinarily proficient experimentalist and eventually participated in disproving his own chain theory (7). The Wernerian Sydern The experimental facts finally were correctly interpreted by Alfred Werner in Ziirich in 1893. Werner recognized with amazing insight that metals in coordination com~oundshave two kinds of valency. He expressed this concept in the following words (8):
Even when, to judge bythe valencenumber,the camhiningpawerof certain atoms is exhausted, they still pmsess in most eases the power of participating further in the construction of complex molecules with the formation of vervdefinite atomic linkaees. The oossibilitv of this action is to be traced back to the fact that.besides
