The whole edifice of substitutive nomenclature is based on concept of parent structures, most importantly parent hydrides. Implicit in parent hydrides are the valencies, or bonding numbers, of non-hydrogen atoms. The standard bonding numbers of neutral atoms in parent hydrides are given thus [1]:
| Bonding number n | 3* | 4 | 3 | 2 | 1 |
|---|---|---|---|---|---|
| B | C | N | O | F | |
| Al | Si | P | S | Cl | |
| Ga | Ge | As | Se | Br | |
| In | Sn | Sb | Te | I | |
| Tl | Pb | Bi | Po | At |
These bonding numbers correspond to the number of hydrogen atoms in mononuclear hydrides for elements of Group 13 to Group 17.
Ah, if we only had to name compounds containing just carbon, hydrogen and oxygen. As soon as we move beyond, the trouble starts. Consider phosphorus. We know from school chemistry that this element has valencies of 3 and 5. For trivalent phosphorus compounds we can create substitutive names based on the parent hydride PH3, phosphane (a). But how to deal with pentavalent phosphorus?
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| (a) | (b) |
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