Nomenclature Free valences

The simplest molecular orbital method to use, and the one involving the most drastic approximations and assumptions, is the Huckel method. One str ength of the Huckel method is that it provides a semiquantitative theoretical treatment of ground-state energies, bond orders, electron densities, and free valences that appeals to the pictorial sense of molecular structure and reactive affinity that most chemists use in their everyday work. Although one rarely sees Huckel calculations in the resear ch literature anymore, they introduce the reader to many of the concepts and much of the nomenclature used in more rigorous molecular orbital calculations. 

The prefix sila- designates replacement of carbon by silicon in replacement nomenclature. Prefix names for radicals are formed analogously to those for the corresponding carbon-containing compounds. Thus silyl is used for SiH3—, silyene for —SiH2—, silylidyne for —SiH<, as well as trily, tetrayl, and so on for free valences(s) on ring structures. 

Where a chelating ligand is formed by removing two or more hydrogen atoms from a parent compound, the atoms with free valencies, understood to form the bonds to the central atoms, are indicated by using the appropriate ligand name (such as propane-1,3-diyl), cf. Section IR-10.2.3. This is demonstrated in Examples 1-3 below. Note that an alternative nomenclature for such metallacycles is currently being developed. 

Standard nomenclature is well known, but it may be noted that (a) iso- (to denote terminal CHMe2) is restricted to C3-C6, t- (tertiary) to C4 and C5, and s- (secondary) to C4, (b) such modified names may never be used for substituted derivatives of the compounds or radicals, e.jf., CHMe2 CHCl- is 1-chloro-2-methylpropyl, and (c) the free valency of radicals always has number 1, Pr°2CH- is 1-propylbutyl and not 4-heptyl. 

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