Valence model, four-state

In our theoretical formulation for PCET [26, 27], the electronic structure of the solute is described in the framework of a four-state valence bond (VB) model [41]. The most basic PCET reaction involving the transfer of one electron and one proton may be described in terms of the following four diabatic electronic basis... 

ISP contain four basic core structures which have been characterized crystallographically both in model compounds and in ISP (Rao and Holm, 2004). These are (Figure 13.15), respectively, (A) rubedoxins found only in bacteria, in which the [Fe-S] cluster consists of a single Fe atom bound to four Cys residues—the iron atom can be in the +2 or +3 valence (B) rhombic two-iron-two-sulfide [Fe2-S2] clusters—typical stable cluster oxidation states are +1 and + 2 (the charges of the coordinating cysteinate residues are not considered) ... 

A simple orbital pieture can be used to deseribe these distortions. The lone pair ean oeeupy either an sp hybridized orbital or an unhybridized s orbital. Around eations from the upper half of the periodie table it usually oeeupies an sp orbital where the four valence-shell eleetron pairs (three bonding and one lone pair) are arranged at the corners of a tetrahedron giving rise to three primary bonds as described by the VSEPR model. Cations from lower in the table ean also show this configuration but are sometimes found with the lone pair oeeupying a pure s orbital which can then be treated as part of the core, giving a spherieally symmetric electron density. Intermediate states of hybridization are also possible and frequently found. 

It is inferred that the affinity state of these doubly ligated species is different from that of fully ligated Hb, but is perhaps an intermediate affinity state. We do not understand why the cooperativity of these intermediate species is greatly reduced. The symmetric cyanomet valency hybrid Hbs [(a+CNp)2) and (a0+CN)2] have also been found to have reduced cooperativity (n = 1.1-1.2) and increased affinity for 02 compared to Hb A (Cassoly and Gibson, 1972 Maeda et al., 1972 Hofrichter et al., 1985). Thus, none of the four models for doubly ligated species can represent the features of K3 and of cross-linked Hb or Hb A, as pointed out by Miura et al. (1987). 

Cp2Zr(CH3)(THF)]+ The zirconium oxidation state is 4+ and each Cp ligand donates six electrons. The ligand CTC donates two electrons. The solvent molecule, THF, also donates two electrons, and the total electron count is 12 + 0 + 2 + 2=16. With the covalent model zirconium is in the zero oxidation state and has four electrons Ad2,5s2) in the valence shell. Both Cp and CH3 are considered as radicals and therefore donate five and one electron, respectively. The valence electron count is therefore 4 + 2x5 + 1+ 2-1 = 16. Notice that because of the positive charge, we subtract one electron. 

---