Section 9.5.3.1. Ordering of entries: the `Bond' column

For an element pair X—Y, the primary ordering is alphabetic by element symbols according to the rows of Fig. 9.5.3.1(a); i.e. X changes slowest, Y fastest. The complete sequence runs from As—As to Te—Te with bonds involving carbon in the natural position: As—CC—CC—Te. Within a given X—Y pair, a secondary ordering is based on the coordination numbers (j) of X and Y, and on the nature of the bond between them. The bond definition is of the form X(j)—Y(j), with j decreasing fastest for Y, slowest for X, and with all single bonds preceding any multiple bonds. For carbon, the formal hybridization state replaces (but is equivalent to) the coordination number and it is for this element that the ordering rules are most clearly required. The ordering of the most populous C—C, C—N, C—O sections is illustrated in Fig. 9.5.3.1(b). The 13 possible C—C combinations follow the sequence Csp³—Csp³, Csp³—Csp², Csp³—C_ar, Csp³—Csp¹, Csp²—Csp², Csp²—C_ar, Csp²—Csp¹, C_ar—C_ar, C_ar—Csp¹, Csp¹—Csp¹, Csp²=Csp², C_arC_ar, Csp¹≡Csp¹. The symbol C_ar represents aryl carbon in six-membered rings, which is treated separately from Csp² throughout the table. The symbol is used to indicate a delocalized double or aromatic bond according to context.