Skeleton molecular orbitals

A qualitative discussion of the theoretical basis for the description of bonding in boranes and carboranes was carried out in Chapter 2. There are three orbitals per boron atom disposable for the formation of the cluster skeleton. Thus, for a given member of the series of the polyhedra derived from the icosahedron illustrated in Fig. 2.27, the linear combination of these boron orbitals gives raise to a set of (n + 1) low-lying skeleton molecular orbitals, n being the nuclearity of the polyhedron. Since each boron atom in boranes distracts one electron in the formation of one two-center/two electron exo -bond with hydrogen, there are only two electrons per boron atom available for... 

In spite of the success of this method it was later felt that the calculation of the charge distribution in conjugated r-systems should be put on a less empirical basis. To achieve this, a modified Huckel Molecular Orbital (HMO) approach (Section 7.4) was developed. Again, the charge distribution in the r-skeleton is first calculated by the PEOE method. 

Molecular Orbital. A function made of contributions of Atomic Orbitals and delocalized throughout the entire molecular skeleton. 

The delocalization of molecular orbitals lies at the heart of modern chemistry. The concept that the tt orbitals of benzene or naphthalene cover the entire carbon skeleton promoted the successful understanding of conjugated molecules. The work of R. Hoffmann and others has proven that in saturated molecules [Pg.1]

Another example which illustrates beautifully the mixing of a group orbitals to form delocalized molecular orbitals is benzene. First of all the six crcc bond orbitals interact to give six linear combinations which are delocalized over the entire carbon skeleton. The amplitudes of the various bond orbitals in each [Pg.23]

In a series of studies of the spectroscopy and photochemistry of nickel(O) -a-diimine complexes, the structural differences among the complexes NiL2 and Ni(CO)2L (L Q-diimine) have been examined by means of molecular orbital calculations and electronic absorption Raman resonance studies.2471, 472 Summing up earlier work, the noninnocence of a-diimine ligands with a flat — N=C—C=N— skeleton in low-valent Ni chemistry and the course of substitution reactions of Ni° complexes with 1,4-diaza-1,3-dienes or a,a -bipyridine have been reviewed.2473... 

Note, however, that even in such a simple case as this, molecular orbitals do not correspond one-to-one with bonds. For example, the highest-energy a orbital in acetylene is clearly made up of both CC and CH bonding components. The reason, as pointed out in Chapter 2, is that molecular orbitals are written as linear combinations of nuclear-centered basis functions, and will generally be completely delocalized over the entire nuclear skeleton. 

In Chapter 7 molecular orbital theory was introduced and discussed with emphasis on its application to organic molecules in which n systems extending over planar skeletons constitute the major problem. We now turn to inorganic compounds of two major types and to one class of organometallic compounds. 

The first step is to find a set of molecular orbitals. Let us consider two nuclei, A and B, each with a positive charge and separated by a distance R (Figure 2-1). In this skeleton of nuclei we allow an electron to move. 

Hiickel molecular orbitals in porphin were investigated by Longuet-Higgins et al. (68), and the extended Hiickel molecular orbital model was applied to metalloporphyrins in attempts by Pullman et al. (93), Ohno et al. (86), and Zerner et al. (120) to explain various experimental observations. Let us briefly consider a description of cyanoferriporphin. According to the Hiickel theory all but the -orbitals of each carbon and nitrogen atom of porphin are used up to form the relatively inert skeleton of single bonds. To describe the -bonding twenty-four molecular orbitals of porphin can then be formed as linear combinations of... 

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