General AB Molecules

2-19 GENERAL AB MOLECULES We shall now describe the bonding in a general diatomic molecule, AB, in which B has a higher electronegativity than A, and both A and B have s and f valence orbitals. The molecular-orbital energy 

The ground-state electronic configuration for BN is ( a y(as y (jCie,fyy( z ) This gives a ir state and a prediction of two bonds 

The BN molecule is thus electronically similar to C2. The bond lengths of C2 and BN are I.3I and 1.28 A, respectively. The BN bond energy is only 92 kcal/mole, as compared to 150 kcal/mole 

The CO, NO+, and CN molecules are isoelectronic with N2, having a ground state. The configuration 

The electronic configuration of NO is Co s ) (crs ) (xj,/) (o-j ) (irx,y )t giving a V ground state. Since the eleventh electron goes into a IT orbital, the number of bonds is now 1, or less than for NO+. The bond length of NO is 1.15 A, longer than either the CO or NO+ distances. The bond energy of NO is 162 kcal/mole, considerably less than the CO value. 

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Figure 2-24 Relative orbital energies i a general AB molecule, with B more electronegative than A.

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