Antibonding effect

Despite the given formula, the calculation of the bond order is not always clear in the case of the occupation of orbitals having only a minor bonding or minor antibonding effect should they be counted or not Nevertheless, the bond order is a simple and useful concept. In valence-bond formulas it corresponds to the number of bonding lines. 

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. 

The overlap is depicted in Fig. 2-4. The bonding strength generated from the overlap between the +s AO and the + portion of the p orbital is canceled by the antibonding effect generated from overlap between the + r and the - portion of the p. The MO is nonbonding (n) it is no better than two isolated AO s. 

Of the 33 excited states of the K molecules that have been reported,1 20 have iuternuclear distance within 0.03 A of the value for HJ", 1.06 A it is therefore probable that these states can be described as corresponding to an ILj ion, with a one-electron bond, and a second electron in an outer orbit, with only a small bonding or antibonding effect. 

The antibonding effect is also apparent if we perform MO calculations on hypothetical molecules such as F35 and Cl35. In the latter case, inclusion of the 3d orbitals exerts a small buffering influence but with a relatively large number of valence electrons compared with the number of valence orbitals, the HOMO is still strongly antibonding. 

Hj, since antibonding effect is greater than bonding effect. 

In conclusion, the energy levels of the d-orbitals in a metalporphy-rin are parametrized including the 7r-antibonding effect as follows ... 

Figure 14 shows the MO correlation diagram between [Pt4(HCOO)s] and the fragment, where only levels near the HOMO-LUMO level associated with Pt da orbitals are depicted. For clarity, levels associated with Pt d orbitals are omitted in Fig. 14, because low-lying d orbitals in the fragment are fully occupied therefore, bonding and antibonding effects due to MOs derived from the d orbitals are compensated and have no net contribution to stability of the whole [Pt4(HCOO)al molecule or the formation of Pt—Pt bonds in the cluster core. 

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