Valence bond theory Bonding bonds

In the valence bond theory, sigma bonds overlap on a line drawn between the two nuclei, while pi bonds result from the overlap of atomic orbitals above and below a line connecting the two atomic nuclei. 

Valence Bond Theory 157 Bond Length and Bond Energy 158 Lewis Structures 159 VSEPR Theory 161... 

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

Usually the bond order corresponds to the number of bonds described by the valence bond theory. Eractional bond orders exist in species that contain an odd number of electrons, such as the nitrogen oxide molecule, NO (15 electrons) and the superoxide ion, 02 (17 electrons). 

In Figure 4.6b we illustrate how the tetrahedral structure of CH4 relates to a cubic framework. This relationship is important because it allows us to describe a tetrahedron in terms of a Cartesian axis set. Within valence bond theory, the bonding in CH4 can conveniently be described in terms of an sp valence state for C, i.e. four degenerate orbitals, each containing one electron. Each hybrid orbital overlaps with the li atomic orbital of one H atom to generate one of four equivalent, localized 2c-2e C—H (T-interactions. 

Chapter 4 Valence bond theory multiple bonding in polyatomic molecules 105... 

Valence bond theory pictures bonding in complex ions as arising from coordinate covalent bonding between Lewis bases (ligands) and Lewis acids (metal ions). Ligand lone pairs occupy hybridized metal-ion orbitals to form complex ions with characteristic shapes. 

Valence bond theory. Valence bond (VB) theory assumes that atoms form covalent bonds as they share pairs of electrons via overlapping valence shell orbitals. A single covalent bond forms when two atoms share a pair of electrons via the sigma overlap of two atomic orbitals—a valence orbital from each atom. A double bond forms when two atoms share two pairs of electrons, one pair via a sigma overlap of two atomic orbitals and one via a pi overlap. A triple bond forms by three sets of orbital overlap, one of the sigma type and two of the pi type, accompanied by the sharing of three pairs of electrons via those overlaps. (When a pair of valence shell electrons is localized at only one atom, that is, when the pair is not shared between atoms, it is called a lone or nonbonding pair.)... 

COVALENT BONDiNG AND ORBiTAL OVERLAP We recognize that eiectrons are shared between atoms in a covaient bond, in valence-bond theory, the bonding eiectrons are visuaiized as originating in atomic orbitais on two atoms. A covaient bond is formed when these orbitais overiap. 

Sigma (a) bond In valence-bond theory, a bond characterized by overlap of a half-fllled orbital of one atom with a half-fllled orbital of a second atom along a line connecting the two nuclei. 

In the valence-bond theory, covalent bonds are visualized as resulting from the overlap of atomic orbitals, resulting in two general t) es of bonds, sigma bonds and pi bonds. 

Valence bond theory views bonding as arising from electron pairs localized between adjacent atoms. These pairs of electrons create bonds. Further, organic chemists commonly use the atomic orbitals involved in the three hybridization states of atoms sp, sp, and sp) to create the orbitals that hold these electrons because doing so allows the resulting orbitals to match the experimentally determined geometries around the atoms. Therefore, hybridization is also a VB theory concept. But how do we make the orbitals that contain the electrons and that reside between adjacent atoms This is where we return to MO theory. 

In the combined molecular orbital theory/valence bond theory approach, bonding in organic molecules is thought of as the in-phase addition (overlapping to create bonding orbitals) and out-of-phase addition, also referred to as subtraction (to create antibonding orbitals) of the hybridized (and possibly any unhybridized 2p) atomic orbitals on adjacent atoms. 

Accx)rding to valence bond theory, a bond forms between two atoms when the following conditions are met ... 

Applying Valence Bond Theory (Multiple Bonding)... 

According to valence bond theory, bonds form between atoms when atomic orbitals overlap, thus allowing the atoms to share valence electrons. A bond forms, furthermore, when the resulting molecule is lower in energy than the original, isolated atoms. 

In valence bond theory, rr bonds are always accompanied by a cr bond. Can the same also be said of the molecular orbital theory for diatomic molecules ... 

Oxygen is a colourless gas which condenses to a pale blue liquid, b.p. 90 K, which is markedly paramagnetic indicating the presence of unpaired electrons (p. 229). Simple valence bond theory (as used in this book) would indicate the structure... 

One widely used valence bond theory is the generalised valence bond (GVB) method of Goddard and co-workers [Bobrowicz and Goddard 1977]. In the simple Heitler-London treatment of the hydrogen molecule the two orbitals are the non-orthogonal atomic orbitals on the two hydrogen atoms. In the GVB theory the analogous wavefunction is written ... 

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... 

T orbital for benzene obtained from spin-coupled valence bond theory. (Figure redrawn from Gerratt ], D L oer, P B Karadakov and M Raimondi 1997. Modem valence bond theory. Chemical Society Reviews 87 100.) figure also shows the two Kekule and three Dewar benzene forms which contribute to the overall wavefunction Kekuleform contributes approximately 40.5% and each Dewar form approximately 6.4%. 

Gerratt J, D L Cooper, P B Karadakov and M Raimondi 1997. Modem Valence Bond Theory. Chemical Society Reviews pp. 87-100. 

---