Four-state valence bond model

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... 

From the valence bond point of view, formation of a complex involves reaction between Lewis bases (ligands) and a Lewis acid (metal or metal ion) with the formation of coordinate covalent (or dative) bonds between them. The model utilizes hybridization of metal s, p, and d valence orbitals to account for the observed structures and magnetic properties of complexes. For example, complexes of Pd(ll) and Pt(Il) are usually four-coordinate, square planar, and diamagnetic, and this arrangement is often found for Ni(II) complexes as well. Inasmuch as the free ion in the ground state in each case is paramagnetic (d, F), the bonding picture has to... 

Of course, the octet is usually not actually violated. Multicenter bonding models require some MOs that are essentially nonbonding and concentrated only on the substituents, and thus, the number of electrons in the valence shell of the central atom rarely exceeds the octet. However, here we should distinguish, between what Musher [61] more than 40 years ago termed hypervalent compounds of first and second kind, respectively. In the first class, the central atom is not in its maximum oxidation state, and thus, the central-atom s-character concentrates in a Ip. Then, as we have discussed in detail above, the bonds are made mainly from np-orbitals of the central atom, and thus, the assumptions of the usual three-center-four-electron bonding models are nicely fulfilled. In contrast, hypervalent compounds of the second kind exhibit the maximum oxidation state and, thus, necessarily involve the ns-orbitals fuUy in bonding. One thus sees (i) extensive hybridization defects... 

A model of carbon bonding established to account for the tetrava-lent nature of carbon involves two steps, in which the atoms are first raised to excited states (valence states), and are then permitted to interact in these states to form the molecule. In the case of carbon, one of the 2s orbitals is thus "promoted" to a p orbital to make available four unpaired electrons. The result is the "hybridization" of an s orbital with three p orbitals to form four equivalent and tetrahedrally oriented sp orbitals. To devise molecular orbitals, one takes a linear combination of atomic orbitals. Following the valence bond theory, the atoms are in their excited state when hybridized, and then come together to form a molecule. In the molecular orbital approach, when the proper coefficients for the wave functions of the linear combination of atomic orbitals are chosen—i.e., those coefficients which will minimize the energy of the resultant molecule—the results will be the same as when hybrid orbitals are employed. Thus, both approaches lead to a minimization of energy and to stable bond formation. 

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. 

Selenium is normally two-fold coordinated, Se, , and of its four valence p electrons, two are bonding electrons and the other two are lone pair states which make up the top of the valence band. A simple molecular orbital model of the bonding is shown in Fig. 4.6. The neutral selenium dangling bond, SCj , has one bonding electron, and... 

Methylene is the simplest example of a carbene, a molecule containing a carbon formally bearing only six valence electrons. Of these, four electrons are involved in the C-H bonds. The orbital occupation of the last two electrons defines the specific electronic state of methylene. If we assume a bent structure, we can use the simple model of an sp -hybridized carbon. The four bonding electrons occupy two of these sp hybrids. This leaves the third sp hybrid (3ai) and the p-orbital (l i) available for the last two electrons (see Figure 5.1). Placing one electron in each of these orbitals with their spins aligned creates a triplet state. The electronic configuration of this triplet state is... 

---