Spin-forbidden reactions in transition metal chemistry

In 2003, we presented two reviews concerning spin-forbidden reactions in transition metal chemistry (10,16). A number of other reviews in 2004 addressed closely related topics (90,91). However, since then, we are not aware of any review on this topic. We, therefore, present a short discussion of a number of recent studies of reactivity in transition metal chemistry for processes involving changes in spin-state. The aim here is not to be exhaustive, as there are now quite a lot of studies of transition metal chemistry that address the topic of spin-state changes. Instead, this is an attempt to give a flavor of the types of question that are being addressed now and the techniques used to investigate them. 

Spin-Forbidden Reactions in Transition Metal Chemistry... 

Dekker, New York, 2001, p. 291. Spin-Forbidden Reactions in Transition-Metal Chemistry. 

In this section, I will try to illustrate the role of spin-forbidden reactions in the chemistry of transition metals. The section is intended to give some idea of the breadth of this chemistry doing justice to the depth would be impossible. 

After describing why there is a problem, I will briefly summarize the theoretical description of spin-forbidden reactions. It will be useful at this point to draw parallels with other types of nonadiabatic chemistry, in particular, electron transfer. Then I will review some of the typical contexts in which spin-forbidden behavior occurs in transition metal systems, to try to illustrate how widespread it is. This will be followed by a presentation of strategies used for characterizing and understanding spin-forbidden reactions, based on the use of energies and... 

The problem referred to in the title of this section is that locating the regions where PESs of the reactant and product spin states approach one another is difficult within computational ab initio quantum chemistry. Also, the way in which the two factors just mentioned combine to determine the rates of spin-forbidden reactions has sometimes been ill appreciated. Taken together, these facts have meant that computational studies have not contributed to understanding the mechanistic role of spin changes in quite the same way that they have helped to clarify the mechanisms of many other reactions of transition metal systems. 

---