Bond valence in Pn species

The bonding in P species can be expressed by their bond valence b, which corresponds to the number of P-P bonds. Let g be the total number of valence electrons in P . When a covalent bond is formed between two P atoms, each of them gains one electron in its valence shell. In order to satisfy the octet rule for P , j( 8n — g) electron pairs must be involved in bonding between the P atoms. The number of these bonding electron pairs is defined as the bond valence b of the P species  

In these P species, the b value is exactly equal to the bond number in the structural formula, as shown in Figs. 15.3.1, 15.3.4, and 15.3.5. But for the 

Structure of transition-metal complexes ofP As3-, andSb3 (a) [P7Cr(CO)3]3-, (b) [As7Mo(CO)3]3- 

The bond valence of a P group changes with its number of valence electrons. The P4 species is a good example, as shown in Fig. 15.3.8. In these structures, each transition metal atom also conforms to the 18-electron rule. 

The bonding structure of transition-metal complexes with P group can be classified into four types  

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