Triplet surface

In total, there are three barrierless entrance pathways frontside and backside addition of hydrogen atom to vinyl, and abstraction of the cis hydrogen. All the pathways discussed so far are on the singlet PES. There are also addition pathways on the triplet surface, but these are impeded by small barriers and are not expected to be important compared to the singlet surface. 

Based on this tentative assignment of Fe(CO)3(C2H4) as having a singlet ground state, addition of ethylene to Fe(CO)3 now becomes a spin-forbidden process, which might seem inconsistent with the observed rapid addition. However, as mentioned above, addition on the triplet surface should be barrierless, and hence rapid. Conversion of the initially formed triplet adduct to the... 

Given the advantages of the -pathway for H-atom abstraction mentioned above, it seems sensible to ask whether a c-pathway could be operative on the triplet surface as well. This possibility was explored for model iron(IV)-oxo complexes involving... 

The energetics and dynamics of the triplet surface remain subject to controversy. Saltiel et al. (21) have recently proposed that triplet energy transfer to either t-1 or c-1 yields a common twisted triplet 3p which is more stable than 3t by 2.1 kcal/mole and thus predominates at equilibrium. Other workers have favored a more balanced 3p 3p equilibrium (22,... 

The reactions, 0(3P) + CS and 0(3P) + C Se, are very similar in their dynamics [450—452]. They are both direct reactions producing CO in high vibrational states with inverted population distributions ( 80 and 70%, respectively). The similarity of the dynamics is illustrated by the virtual identity of the CO vibrational population distributions for the two reactions when plotted against Fv [452]. It seems most likely in view of the direct nature of the reactions that they take place on a repulsive triplet surface without a crossing to the lower singlet surface corresponding to the stable species, OCS or OCSe. 

Figure 14 Schematic representation of solvation/spin-crossover process. The short horizontal bars in the unsolvated triplet surface denote vibrational levels. 

Concise theoretical studies of Ziegler145,146 analyzed all of the possible reaction pathways including the crossover from the singlet to the triplet surface with the transition state on the singlet surface while the formed product is a triplet species. It could be shown that the epoxide precursor is formed via a [3 + 2]-addition of ethylene to two Cr=0 bonds followed by rearrangement to the epoxide product (Scheme 10). 

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