Covalent Bond Lengths and Interatomic Distances

The experimental value is usually shortened from the covalent radius sum, and this may arise from (a) partial ionic character of the bond, or (b) the occurrence of k bonding. Unless the bond is 

FIGURE 3.13 Tentative classification of interatomic distances. Increasing lengths from left to right. 

Any bond shortening beyond that expected from electronegativity difference is probably (but not necessarily) attributable to rr character of the bond. Assuming this to be so, data shown in Table 3.17 can be calculated for the amount of n character in various types of bond. 

The tetrahedral symmetry of P0 allows the formation of the strong n bonding orbitals with Sd sy and 3dj2 of P with 2p7t of each O atom. In this valence bond language which allows a total 7t bond order of 2, each P-O bond has a 7t bond order of 1/4 + 1/4 = 1/2. 

Point A corresponds to the P-0 distance of 1.54 A in the PO anion with a BO of 1/2, while point B corresponds to the single bond distance of 1.71 with BO = 0. 

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