Hybridization a scheme for trigonal planar species

The notation sp means that one and two p atomic orbitals mix to form a set of three hybrid orbitals with different directional properties. 

Let us consider the combination of 2s, 2p and 2py atomic orbitals. The final hybrid orbitals must be equivalent in every way except for their directional properties sp hybrids must contain the same amount of s character as each other and the same amount of p character as one another. We begin by giving one-third of the 2s character to each sp hybrid orbital. The remaining two-thirds of each hybrid orbital consists of 2p character, and the normalized wave-functions are given in equations 4.3 to 4.5. 

The model of sp hybridization can be used to describe the (T-bonding in trigonal planar molecules such as BH3. The valence state of the B atom is sp Y (i.e. three sp hybrid orbitals, each with one electron) and the equivalence of the B—H interactions follows by considering that each interaction is formed by the overlap of one B sp hybrid orbital with the I5 atomic orbital of an H atom (Fig. 5.5). Each H atom contributes one electron to the bonding scheme and, so, each B—H cr-bond is a localized 2c-2e interaction (see Section 2.2). A diagram similar to that shown in Fig. 5.3 can be constructed to show the formation of a valence state for the trigonal planar B atom. 

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For molecular species with other than linear, trigonal planar or tetrahedral-based structures, it is usual to involve d orbitals within valence bond theory. We shall see later that this is not necessarily the case within molecular orbital theory. We shall also see in Chapters 14 and 15 that the bonding in so-called hypervalent compounds such as PF5 and SFg, can be described without invoking the use of J-orbitals. One should therefore be cautious about using sp"d hybridization schemes in compounds of />-block elements with apparently expanded octets around the central atom. Real molecules do not have to conform to simple theories of valence, nor must they conform to the sp"d" schemes that we consider in this book. Nevertheless, it is convenient to visualize the bonding in molecules in terms of a range of simple hybridization schemes. 

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