Valency, stereochemical control

Enemark, J. H., Feltham, R. D., Riker-Nappier, J., and Bizot, K. F. (1975). Stereochemical control of valence. IV. Comparison of the structures and chemical reactivities of five-and six-coordinate diarsine complexes of the cobalt nitrosyls CoNO) group. Inorg. Chem. 14, 624-632. 

In their extensive studies of metal nitrosyl chemistry, Enemark and Feltham showed that the mode of NO bonding can be altered by the simple addition of another ligand (168, 204, 205). An example of this phenomenon is illustrated by reaction (83b), and has been described by these investigators as stereochemical control of valence (204). 

Fig. 15.17 Stereochemical control of valence. Note localization of the lone pair on the nitrogen atom and bending of the niirosyl group upon addition of thiocyanate ion to the coordination sphere. Bond lengths are in picometer.s. [From Enemark, J. H. Feltham, R. D. Prof. Nall. Acad. Set. USA 1972, 69, 3534. Used with permission.]...

We can actually see the process of electron pair shift with a resultant change in structure in the complex ion [Co(diars)2NO)J+ (where diars is a bidentale diarsine ligand) (Fig. 15.17). The 18-electron rule predicts that the nitrosyl group will be linear (a three-electron donor), as indeed it is. Reaction of this complex with the thiocyanate ion (a two-electron donor) would violate the 18-electron rule unless a pair is shifted from a molecular orbital of largely metal character to an orbital on nitrogen. This is in fact what happens and stereochemical control of valence" results.47 As NO goes from being a three-electron to a one-electron donor, a coordination site capable of... 

Feltham and Enemark believed that the transformation of linear and bent nitrosyls represented a specific example of a more general phenomenon which they described as the stereochemical control of valency [103]. This term was meant to indicate that the Unear-bent preferences were controlled primarily by the total number of electrons, X. As the years have gone by, the completion of nearly 3,000 structures has shown that the problem is more complex than originally envisaged. The fact that there are many... 

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