Spin state changes

The electrons do not undergo spin inversion at the instant of excitation. Inversion is forbidden by quantum-mechanical selection rules, which require that there be conservation of spin during the excitation process. Although a subsequent spin-state change may occur, it is a separate step from excitation. 

The interconversion between different spin states is closely related to the intersystem crossing process in excited states of transition-metal complexes. Hence, much of the interest in the rates of spin-state transitions arises from their relevance to a better understanding of intersystem crossing phenomena. The spin-state change can alternatively be described as an intramolecular electron transfer reaction [34], Therefore, rates of spin-state transitions may be employed to assess the effect of spin multiplicity changes on electron transfer rates. These aspects have been covered in some detail elsewhere [30]. 

Note The case of Fe2+ is especially complicated by spin-state changes and polymerisation of mixed oxidation states. 

SPIN-STATE CHANGES AND REACTIVITY IN TRANSITION METAL CHEMISTRY REACTIVITY OF IRON TETRACARBONYL... 

We (as well as many others (4-8), for reviews see (9,10)) have been interested for some time (11,12) in the effect of spin-state changes on reactivity. In particular, we have used computational methods to explore these effects in transition metal chemistry ((13-15), for reviews see (10,16)). The key factor affecting reactivity is the relative energy at which the zeroth-order potential energy surfaces corresponding to the individual spin-states cross (Scheme 2). This factor will determine, among other things, whether spin-forbidden reactivity is competitive with... 

Three different mechanisms were considered (i) direct spin-forbidden addition of CO (ii) spin-state change from 3Fe(CO)3 (H2) to 1Fe(CO)3(H2) or 1Fe(CO)3(H)2, followed by rapid addition of CO and (iii) spin-allowed substitution reaction between CO and 3Fe(CO)3(H2) to give 3Fe(C0)4 and H2, followed by spin-forbidden addition of H2 to 3Fe(C0)4. The results are summarized in Table V. 

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. 

Jeremy Harvey thanks the EPSRC for an Advanced Research Fellowship, and acknowledges many helpful conversations on spin-state changes with Helmut Schwarz, Detlef Schroder, Sason Shaik, Massimiliano Aschi, Rinaldo Poli, James Mayer, and many others. A special thanks is due to Prof. Michael George for the collaborative work discussed in the text. Maria Besora thanks the Department of Education and Universities (DEiU) of the Autonomous Government of Catalonia for financial support. JLCM thanks Mexico s National Council of Science and Technology (CONACYT) for a scholarship. 

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