Pauling hybrid-orbital theory

Heterocyclic systems have played an important role in this historical development. In addition to pyridine and thiophene mentioned earlier, a third heterocyclic system with one heteroatom played a crucial part protonation and methylation of 4//-pyrone were found by J. N. Collie and T. Tickle in 1899 to occur at the exocyclic oxygen atom and not at the oxygen heteroatom, giving a first hint for the jr-electron sextet theory based on the these arguments.36 Therefore, F. Arndt, who proposed in 1924 a mesomeric structure for 4//-pyrone, should also be considered among the pioneers who contributed to the theory of the aromatic sextet.37 These ideas were later refined by Linus Pauling, whose valence bond theory (and the electronegativity, resonance and hybridization concepts) led to results similar to Hiickel s molecular orbital theory.38... 

In contrast to the four tetrahedrally oriented elliptic orbits of the Sommer-feld model, the new theory leads to only three, mutually orthogonal orbitals, at variance with the known structure of methane. A further new theory that developed to overcome this problem is known as the theory of orbital hybridization. In order to simulate the carbon atom s basicity of four an additional orbital is clearly required. The only possible candidate is the 2s orbital, but because it lies at a much lower energy and has no angular momentum to match, it cannot possibly mix with the eigenfunctions on an equal footing. The precise manoeuvre to overcome this dilemma is never fully disclosed and appears to rely on the process of chemical resonance, invented by Pauling to address this, and other, problems. With resonance, it is assumed that, linear combinations of an s and three p eigenfunctions produce a set of hybrid orbitals with the required tetrahedral properties. 

A modern model of benzene Since the time of Kekul6 s proposal, research has confirmed that benzene s molecular structure is indeed hexagonal. However, an explanation of benzene s unreactivity had to wait until the 1930s when Linus Pauling proposed the theory of hybrid orbitals. When applied to benzene, this theory predicts that the pairs of electrons that form the second bond of each... 

Qualitative application of VB theory to molecules containing second-row elements such as carbon, nitrogen, and oxygen involves the concept of hybridization, which was deveioped by Linus Pauling. The atomic orbitals of the second-row elements include the spherically symmetric 2s and the three 2p orbitals, which are oriented perpendicularly to one another. The sum of these atomic orbitals is equivalent to four sp orbitals directed toward the corners of a tetrahedron. These are called sp hybrid orbitals. In methane, for example, these orbitals overlap with hydrogen Is orbitals to form CT bonds. 

For most of the molecules and reactions we want to consider, the Pauling hybridization scheme provides an effective structural framework, and we use VB theory to describe most of the reactions and properties of organic compounds. However, we have to keep in mind that it is neither a unique nor a complete description of electron density, and we will find cases where we need to invoke additional ideas. In particular, we discuss molecular orbital theory and density functional theory, which are other ways of describing molecular structure and electron distribution. 

In subsequent independent papers, Pauling [4] and Slater [6] generalized the valence-bond treatment made for the H2 molecule to polyatomic systems as H2O, NH3, CH4 etc. .. where an atom of the first period (the second row) is linked to hydrogens by several two-electron bonds they described the valence orbitals coming from the central atom by appropriate s and p combinations known later as hybrid orbitals. At the same time Hund [7] and Mulliken [8] presented another quantum theory of valence, the molecular orbital method in LCAO form, using the spectroscopic concept of molecular configuration built from s, p, d. ..pure atomic orbitals. The actual status of the hybridization process was clarified by Van Vleck [9], who showed that the various approximations... 

Actually, the recent studies of transition-group spectra have shown that two different models cherished by many theorists— the electrostatic ligand field model and the valency-bond description (more specifically, the Pauling hybridization theory)—cannot be applied to the observed distribution of energy levels. On the other hand, the molecular orbital (M.O.) configurations give an excellent classification of all energy levels of all complexes and a unified description is obtained of all polyatomic molecules. 

In the 1930s, the development of valence bond theory (most notably by Linus Pauling) was extended to include linear combinations of the valence orbitals themselves. Such linear combinations are called hybrid orbitals. Specifically, the combination of a certain number of atomic orbitals provides linear combination hybrid orbitals that collectively have the proper symmetry. This single fact is what makes valence bond theory and hybrid orbitals such a usefijl interpretational tool in chemistry (whether or not such orbitals actually exist). 

Pauling s valence bond theory is likewise of only limited value in its application to transition metal complexes. In the VBT, the ligand electrons are accommodated in hybrid orbitals localized on the central metal. The orbitals of interest for transition metals are the nd, n -f 1), n + l)p, and n + )d. An octahedral configuration arises from d sp hybridization of the metal orbitals, while dsp hybridization gives the square planar structure and sp hybridization results in a tetrahedral geometry. 

Stork [1963] completely rejects the Russian accusation that resonance theory as developed by Pauling, Wheland, and Ingold fails both empirically and philosophically. However, it should be noted that these criticisms (in the GDR) of the events in Moscow appeared well after the hype was over in the USSR. Furthermore, as Brush [1999, 268] correctly remarks For chemists in the West, the apparent absurdity of the Marxist ideological critique obscured a significant difference between VB and MO. According to Brush the molecular orbital theory permits a more realistic interpretation than the valence bond (or resonance) theory. However that may be, reflection on resonance hybrids, tautomerism, the riddle of benzene, and such like is still a relatively neglected theme in the philosophy of chemistry. 

The fascinating thing about the actual computational developments in this period, however, was that almost none used the so-called valence bond (VB) approach that Pauling had proposed as the foundation of the theory of the bond. The technical reasons for this are well enough known. The non-orthogonality between the hybrid orbitals, a feature essential for the justification of Pauling s approach, made formulating the equations for calculation just too complicated and difficult and. 

The concepts of directed valence and orbital hybridization were developed by Linus Pauling soon after the description of the hydrogen molecule by the valence bond theory. These concepts were applied to an issue of specific concern to organic chemistry, the tetrahedral orientation of the bonds to tetracoordinate carbon. Pauling reasoned that because covalent bonds require mutual overlap of orbitals, stronger bonds would result from better overlap. Orbitals that possess directional properties, such as p orbitals, should therefore be more effective than spherically symmetric 5 orbitals. 

In his valence bond theory (VB), L. Pauling extended the idea of electron-pair donation by considering the orbitals of the metal which would be needed to accommodate them, and the stereochemical consequences of their hybridization (1931-3). He was thereby able to account for much that was known in the 1930s about the stereochemistry and kinetic behaviour of complexes, and demonstrated the diagnostic value of measuring their magnetic properties. Unfortunately the theory offers no satisfactory explanation of spectroscopic properties and so was... 

Pauling, L. Bond Angles in Transition-Metal Tricarbonyl Compounds A Test of the Theory of Hybrid Bond Orbitals Proc. Natl. Acad. Sci. (USA) 1978, 75, 12-15. 

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