Valence bond theory Bonding single bonds

Another approach is spin-coupled valence bond theory, which divides the electrons into two sets core electrons, which are described by doubly occupied orthogonal orbitals, and active electrons, which occupy singly occupied non-orthogonal orbitals. Both types of orbital are expressed in the usual way as a linear combination of basis functions. The overall wavefunction is completed by two spin fimctions one that describes the coupling of the spins of the core electrons and one that deals with the active electrons. The choice of spin function for these active electrons is a key component of the theory [Gerratt ef al. 1997]. One of the distinctive features of this theory is that a considerable amount of chemically significant electronic correlation is incorporated into the wavefunction, giving an accuracy comparable to CASSCF. An additional benefit is that the orbitals tend to be... 

It is not possible to write down a single, satisfactory, classical bonding diagram for S4N4 and, in valence-bond theory, numerous resonance hybrids must be considered of which the following are typical ... 

Valence bond theory does agree fairly well with molecular orbital (MO) theory for homonuclear diatomic molecules that can obey the octet rule H2 (single bond, bond order = 1), Li2 (single bond, bond order = 1), N2 (triple bond, bond order = 3), 02 (double bond, bond order = 2), F2 (single bond, bond order = 1). However, for those molecules that don t, it is more difficult to know if they exist or not and what bond orders they have. MO theory allows us to predict that He2, Be2 and Ne2 do not exist since they have bond orders = 0, and that B2 has bond order = 1 and C2 has bond order = 2. 

Apply VSEPR and MO theory to the carbonate ion, CO,2, and also sketch the canonical forms of the ion which would allow valence bond theory to represent the bonding as accurately as possible. The single bond covalent radii of C and O are 77 and 74 pm, respectively. The C- O bond distance in the carbonate ion is 129 pm. 

Valence bond theory provides an easily visualized orbital picture of how electron pairs are shared in a covalent bond. In essence, a covalent bond results when two atoms approach each other closely enough so that a singly occupied valence orbital on one atom overlaps a singly occupied valence orbital on the other atom. The now-paired electrons in the overlapping orbitals are attracted to the nuclei of both atoms and thus bond the two atoms together. In the H2 molecule, for instance, the H-H bond results from the overlap of two singly occupied hydrogen Is orbitals ... 

Valence bond theory has two main problems (1) For molecules such as 02, valence bond theory makes an incorrect prediction about electronic structure. (2) For molecules such as O3, no single structure is adequate and the concept of resonance involving two or more structures must be added (Section 7.7). The first problem occurs rarely, but the second is much more common. To better deal with resonance, chemists often use a combination of bonding theories in which the <7 bonds in a given molecule are described by valence bond theory and it bonds in the same molecule are described by MO theory. 

Valence bond theory is somewhat out of favour at present a number of the spectroscopic and magnetic properties of transition-metal complexes are not simply explained by the model. Similarly, there are a number of compounds (with benzene as an organic archetype) which cannot be adequately portrayed by a single two-centre two-electron bonding representation. Valence bond theory explains these compounds in terms of resonance between various forms. This is the origin of the tautomeric forms so frequently encountered in organic chemistry texts. The structures of some common ligands which are represented by a number of resonance forms are shown in Fig. 1-11. 

Graphite has layers of hexagonal networks. Each C atom is bonded to three C atoms in the same plane (sp2 hybrids). There is one electron per C in delocalized tt orbitals extending throughout the layer or, in terms of the valence bond theory, there are alternating single and double bonds. Graphite conducts electricity within the plane and can be used as an electrode. 

R. D. Harcourt, in Valence Bond Theory, D. L. Cooper, Ed., Elsevier, Amsterdam, The Netherlands, 2002, pp. 349-378. Valence Bond Structures for Some Molecules with Four Singly-Occupied Active-Space Orbitals Electronic Structures, Reaction Mechanisms, Metallic Orbitals. 

In classical Valence Bond theory, a bond is simply defined as a singlet coupled orbital (electron) pair. Thus, a single bond is obtained using ... 

Many solid-state physicists discuss the structure and properties of metals and alloys with use of the band theory, in its several modifications. This theory is also a quantum mechanical theory, which starts with a solution of the wave equation for a single electron, and introduces electron-electron correlation in one or another of several ways. The resonating-valence-bond theory introduces electron-electron correlation in several stages, one of which is by the formation of covalent bonds between adjacent atoms, and another the application of the electroneutrality principle to restrict the acceptable structures to those that involve only M+, M°, and M-. It should be possible to find a relationship between the band-theory calculations and the resonating-covalent-bond theory, but I have been largely unsuccessful in finding such a correlation. I have, for example, not been able to find any trace of the metallic orbital in the band-theory calculations, which thus stand in contrast to the resonating-valence-bond theory, in which the metallic orbital plays a predominant role."... 

X-ray crystallographic analysis indicated that benzene is a planar, regular hexagon in which all the carbon-carbon bond lengths are 139 pm, intermediate between the single C-C bond in ethane (154 pm) and the C=C bond in ethene (134 pm), and therefore all have some double bond character. Thus the representation of benzene by one Kekule structure is unsatisfactory. The picture of benzene according to valence bond theory is a resonance hybrid of the two Kekule or canonical forms 4 and 9, conventionally shown as in Figure 1.2, and so each carbon-carbon bond apparently has a bond order of 1.5. 

The oxygen molecule contains two more electrons than Ng, and these are in the antibonding vn orbitals. As there are two such orbitals, they are singly occupied by these electrons which have parallel spin. Valence-bond theory fails to account so readily for the paramagnetism. 

We learned in Chapter 5 that each solution to the Schrodinger equation, called a wave function, represents an atomic orbital. The mathematical pictures of hybrid orbitals in valence bond theory can be generated by combining the wave functions that describe two or more atomic orbitals on a single atom. Similarly, combining wave functions that describe atomic orbitals on separate atoms generates mathematical descriptions of molecular orbitals. 

When carbon vaporizes at extremely high temperatures, among the species present in the vapor is the diatomic molecule C2. Write a Lewis formula for C2. Does your Lewis formula of C2 obey the octet rule (C2 does not contain a quadruple bond.) Does C2 contain a single, a double, or a triple bond Is it paramagnetic or diamagnetic Show how molecular orbital theory can be used to predict the answers to questions left unanswered by valence bond theory. 

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