Hybridized orbitals species

The total electron density contributed by all the electrons in any molecule is a property that can be visualized and it is possible to imagine an experiment in which it could be observed. It is when we try to break down this electron density into a contribution from each electron that problems arise. The methods employing hybrid orbitals or equivalent orbitals are useful in certain circumsfances such as in rationalizing properties of a localized part of fhe molecule. Flowever, fhe promotion of an electron from one orbifal fo anofher, in an electronic transition, or the complete removal of it, in an ionization process, both obey symmetry selection mles. For this reason the orbitals used to describe the difference befween eifher fwo electronic states of the molecule or an electronic state of the molecule and an electronic state of the positive ion must be MOs which belong to symmetry species of the point group to which the molecule belongs. Such orbitals are called symmetry orbitals and are the only type we shall consider here. 

Identify the hybrid orbitals used by the atom in boldface type in each of the following species (a) BF3 (b) AsL (c) BrL3 ... 

Triatomic species can be linear, like CO2, or bent, like O3. The principles of orbital overlap do not depend on the identity of the atoms involved, so all second-row triatomic species with 16 valence electrons have the same bonding scheme as CO2 and are linear. For example, dinitrogen oxide (N2 O) has 16 valence electrons, so it has an orbital configuration identical to that of CO2. Each molecule is linear with an inner atom whose steric number is 2. As in CO2, the bonding framework of N2 O can be represented with sp hybrid orbitals. Both molecules have two perpendicular sets of three tt molecular orbitals. The resonance structures of N2 O, described... 

Note that the structures of many of these species are similar to the model structures shown in Figure 4.1. Although the hybrid orbital type is sp3, the structure is characterized as bent or angular, not... 

In this chapter, procedures for drawing molecular structures have been illustrated, and a brief overview of structural inorganic chemistry has been presented. The structures shown include a variety of types, but many others could have been included. The objective is to provide an introduction and review to the topics of VSEPR, hybrid orbitals, formal charge, and resonance. The principles discussed and types of structures shown will be seen later to apply to the structures of many other species. 

Lithiacarboranes are obtained when 1,2,4-triboracyclopentanes and 1,3,5-triboracyclohexanes are reacted with elemental Li in donor solvents. In the contact ion pairs Li ions coordinate to B-B and B-C bonds depending on the number of methylene bridges in bis- and trishomoaromatic species (O, P). The 3c2e bonds in N, O are described as 7i,cr-distorted (indicated as dashed circles) and the overlapping sp3 hybrid orbitals in P practically yield a bonding (dashed triangle). Derivatives of N, O, and P fully characterized by X-ray structure analyses, are presented. 

Before we move on from the hybrid orbitals of carbon, we should take a look at the electronic structure of important reactive species that will figure prominently in our consideration of chemical reactions. First, let us consider carbanions and carbocations. We shall consider the simplest examples, the methyl anion CHs and the methyl cation CH3+, though these are not going to be typical of the carbanions and carbocations we shall be meeting, in that they lack features to enhance their stability and utility. 

VB-MO correspondence for three-electron bonds. The MO description of the ground state of the three-electron bonded species, (R—X)-, is a2a i.e. a doubly occupied a-orbital and a singly occupied a orbital. Substituting the hybrid orbital description of a and a into the MO description of the three-electron bond, we obtain (50). 

The two metal atoms make use of hybrid orbitals as indicated, and each of these overlaps a hydrogen orbital that is centered midway between the metals but substantially above the metal-metal axis. For a dimeric species, such as diborane, this is repeated with a second set of orbitals and hydrogen atom symmetrically placed on the opposite side of the metal-metal axis. Examination of this model shows that each of the terminal bonds is a normal 2-electron-2-center bond. Only the bonds between the metal centers are nonconventional. An extensive review of these systems has been given by Lipscomb (69). 

A similar diagram (II) may be drawn for species that make use of carbon or silicon as the bridging atom. In these systems the orbital used for bridge formation may be described in terms of a hybrid orbital from the bridging atom which becomes five- (or six)-coordinate. 

Fig. 3.26 A tetrahedral AB4 species with vectors representing central atom hybrid orbitals...

In carrying out the procedure for a tetrahedral species, it is convenient to let four vectors on the central atom represent the hybrid orbitals we wish to construct (Fig. 3.26). Derivation of the reducible representation for these vectors involves performing on them, in turn, one symmetry operation from each class in the Td point group. As in the analysis of vibrational modes presented earlier, only those vectors that do not move will contribute to the representation. Thus we can determine the character for each symmetry operation we apply by simply counting the number of vectors that remain stationary. The result for AB4 is the reducible representation, I",. 

In the following tables, the valence state of the central atom is described in terms of orbital occupancy. Thus, for example, h h I p1 denotes a valence state in which two hybrid orbitals (of specified type) are singly occupied (and are used in bond formation) while a third accommodates a lone pair. The singly-occupied pure np orbital forms a p -p bond. Empty hybrid orbitals (denoted h°) always function as acceptor orbitals in the formation of coordinate (or dative) bonds. Doubly-occupied hybrid orbitals (h2) may be lone pairs, or may function as donor orbitals in coordinate bonds, in which case the h is underlined. A doubly-occupied np orbital always forms a dative n bond, while a singly-occupied np orbital always forms an ordinary ji bond. Singly-occupied nd orbitals form d -pn bonds. For the reasons discussed in Section 6.1, you will find few stable species where np orbitals are left empty. 

One way to generate surfaces is by explicit QM calculation of species as they are followed through some mechanism. SCF-CI calculations have proven of considerable value in the author s research. The philosophy here has been to include as basis orbitals only those atomic and hybrid orbitals which are part of chromo-phores or make up bonds which are altered, broken, formed or modified, during the photochemical transformation. Additionally, basis orbitals aimed along the directions of bonds are used, since then the SCF wavefunctions are linear combinations of recognizable orbitals of bonds rather then arbitrary vertically and horizontally oriented atomic orbitals. 

---