Structures and bonding in transition metal complexes

Complexes such as Cu(NH3)62+ have been known and studied since the mid-nineteenth century. Why they should form, or what their structures might be, were complete mysteries. At that time all inorganic compounds were thought to be held together by ionic charges, but ammonia, of course, is electrically neutral. A variety of theories such as 

Werner was able to show, in spite of considerable opposition, that transition metal complexes consist of a central ion surrounded by ligands in a square-planar, tetrahedral, or octahedral arrangement. This an especially impressive accomplishment at a time long before X-ray diffraction and other methods had become available to observe structures directly. His basic method was to make inferences of the structures from a careful examination of the chemistry of these complexes and particularly the existence of structural isomers. For example, the existence of two different compounds AX4 having the same composition shows that its structure must be square-planar rather than tetrahedral. 

An understanding of the nature of the bond between the central ion and its ligands would have to await the development of Lewis shared-electron pair theory, and Pauling s valence-bond picture. We have already shown (Page 50) how hybridization of the d orbitals of the central ion creates vacancies able to accommodate one or more pairs of unshared electrons on the ligands. Although these models correctly predict the structures of transition metals, they are by themselves unable to account for several of their special properties  

Unpaired electrons act as tiny magnets if a substance that contains unpaired electrons is placed near an external magnet, it will undergo an attraction that tends to draw it into the field. Such substances are said to be paramagnetic, and the degree of 

The current model of bonding in coordination complexes developed gradually between 1930-1950, and has largely superseded the hybridization model discussed previously. It is essentially a simplified adaptation of molecular orbital theory which focuses on the manner in which the electric field due to the unpaired electrons on the ligands interact with the five different d orbitals of the central ion. 

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A new look at structure and bonding in transition metal complexes. J. K. Burdett, Adv. Inorg. Chem. Radiochem., 1978, 21,113-146 (73). 

A New Look at Structure and Bonding in Transition Metal Complexes Jeremy K. Burdett... 

A NEW LOOK AT STRUCTURE AND BONDING IN TRANSITION METAL COMPLEXES... 

R. Mason and G. Wilkinson Structure and bonding in transition metal complexes of some cyclic unsaturated ligands, pp. 233-239 (15). 

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