Valence-Bond Structures for Some Diatomic Molecules

Abstract

We shall now generate valence-bond structures for some diatomic molecules and ions that involve atoms of first-row and second-row elements. To do this we shall use their molecular orbital configurations together with the prototype valence-bond structures of Table 3–2. Green and Linnett¹ were the first workers to adopt this approach to the construction of valence-bond structures, and they have described many of the valence-bond structures that we shall generate here. We shall restrict our attention to the molecular orbitals that are constructed from the valence-shell 2s and 2p or 3s and 3p atomic orbitals, and for simplicity in the molecular orbital notation, neglect any hybridization that may occur between s and pa (≡ p_z) atomic orbitals. In Figs. 4–1 and 4–2 we display schematic contours for the molecular orbitals that may be constructed from 2p atomic orbitals, and the n = 2 molecular orbital energy levels for homonuclear diatomic molecules. To construct the ground- state molecular orbital configuration, we feed the electrons into the lowest-energy molecular orbitals and restrict the maximum orbital occupancy to’ two electrons. If only two electrons are to be allocated to a pair of degenerate molecular orbitals (for example, the antibonding π ^*_x and π ^*_y molecular orbitals of O₂), then the lowest- energy arrangement for these electrons occurs when each orbital is singly occupied with parallel spins for the two electrons.