Molecular bonds/orbitals bonding/antibonding/nonbonding

Ti02, there is much more than a guess at a DOS. There is a chemical characterization of the localization in real space of the states (are they on Pt on H on Ti on O ) and a specification of their bonding properties (Pt-H bonding, antibonding, nonbonding, etc.). The chemist asks right away, where in space are the electrons Where are the bonds There must be a way that these inherently chemical, local questions can be answered, even if the crystal molecular orbitals, the Bloch functions, delocalize the electrons over the entire crystal. 

First, we focus on electrons associated with the cr bond framework and determine hybridization schemes for the atoms. Then, we determine the number of electrons in the tt system, and the number of 2p orbitals involved forming the tt molecular orbitals. The key to solving this problem is reasoning out the number of each type of molecular orbital (bonding, antibonding, nonbonding) in the tt system. The final step is to assign electrons in the tt system to the appropriate molecular orbitals. 

All lone pair orbitals have a node between the two atoms and, hence, have a slightly antibonding character. This destabilizing effect of the lone pair localized molecular orbitals corresponds to the nonbonded repulsions between lone pair atomic orbitals in the valence bond theory. In the MO theory all bonding and antibonding resonance effects can be described as sums of contributions from orthogonal molecular orbitals. Hence, the nonbonded repulsions appear here as intra-orbital antibonding effects in contrast to the valence-bond description. 

For example, by combining the previous statements, Bingham formulates the following rule. Electron delocalization will, therefore, stabilize trans conformations relative to the corresponding cis structures when only bonding or nonbonding molecular orbitals are occupied. Cis conformations will be stabilized (less destabilized) relative to trans when antibonding molecular orbitals are also filled. ... 

Fio. 10. The molecular orbitals of the allyl group a (tli bonding), b nonbonding), and c 1/13 antibonding), and d, the metal d orbital overlap with ijif... 

This model implies that the TBP M atom could use its nsp2 orbitals for bonding with the equatorial ligands to form two-center bonds, and its npz orbital could be involved in the interaction with an appropriate orbital of the axial substituent X and a lone electron pair of the donor atom to form a hypervalent, 3c-4e bond in the axial moiety D — M—X. The simplest MO diagram of a 3c-4e bond may be represented by three molecular orbitals bonding (b), nonbonding (nb) and antibonding (ab) (Scheme 1). 

The primary difference between covalent and ionic bonding is that with covalent bonding, we must invoke quantum mechanics. In molecular orbital (MO) theory, molecules are most stable when the bonding MOs or, at most, bonding plus nonbonding MOs, are each filled with two electrons (of opposite spin) and all the antibonding MOs are empty. This forms the quantum mechanical basis of the octet rule for compounds of the p-block elements and the 18-electron rule for d-block elements. Similarly, in the Heider-London (valence bond) treatment... 

Orbitals that include more than one atom in a molecule. Molecular orbitals can be bonding, antibonding, or nonbonding, (p. 669)... 

---