Digonal

All three M+ ions are known to form compounds with the unusual digonal linear coordination (see also section 4.9.7), though this is most common for gold. As a result, complexes R3PMX of copper and silver are normally di-and tetranuclear species with 3- or 4-coordinate metals, while the gold analogues are 2-coordinate monomers. This is the reverse of what would be expected on steric grounds [13, 14],... 

Figure 4.1 Mixing of atomic orbitals to give hybrid orbitals capable of generating digonal 2-coordination. (From J.E. Huheey, Inorganic Chemistry, Harper and Row, London, 1975. Reprinted by permission of Addison-Wesley Educational Publishers Inc.)...

The most important complexes with C-donors, other than organometallics, are cyanides. AgCN has a structure with Ag—C-N-Ag bonding in linear polymeric chains it dissolves in excess KCN forming K+Ag(CN)2 (digonal with Ag-C 2.13A, i[Pg.288]

The dithioacetate is a tetramer, still with digonally coordinated gold [100]. Though long known, the gold complexes of terpenethiolates ( liquid gold )... 

It certainly does not seem that these interactions continue in solution, so that their magnitude is weaker than solvation forces. Theoretical explanation has suggested that the unused, filled, 6s-5dz2 hybrid (section 4.1) interacts with vacant 6px,py orbitals at right angles to the digonal bonds (Figure 4.49). 

Figure 3.17 Geometry of hybrid orbitals, (a) digonal sp hybrids oppositely directed along the same axis (b) trigonal sp2 hybrids pointing along three axes in a plane inclined at 120° (c) tetrahedral sp3 hybrids pointing towards the comers of a regular tetrahedron. (Reproduced with permission from R. McWeeny, Coulson s Valence, 1979, Oxford University Press, Oxford.)...

There exists no uniformity as regards the relations between localized orbitals and molecular symmetry. Consider for example an atomic system consisting of two electrons in an (s) orbital and two electrons in a (2px) orbital, both of which are self-consistent-field orbitals. Since they belong to irreducible representations of the atomic symmetry group, they are in fact the canonical orbitals of this system. Let these two self-consistent-field orbitals be denoted by Cs) and (2p), and let (ft+) and (ft ) denote the two digonal hybrid orbitals defined by... 

Both pyrrolizidines and indolizidines may be similarly formed by cyclization at the less-substituted, internal position of trialkyl-substituted al-kene radical cations (Scheme 29) [139,143]. Related processes featuring exo-digonal radical cyclizations have also been described (Scheme 30) [139,141 — 143]. 

Scheme 30 Nucleophilic trapping followed by exo-digonal radical cyclization...

Similar, but different, redeployment is envisaged when a carbon atom combines with three other atoms, e.g. in ethene (ethylene) (p. 8) three sp hybrid atomic orbitals disposed at 120° to each other in the same plane plane trigonal hybridisation) are then employed. Finally, when carbon combines with two other atoms, e.g. in ethyne (acetylene) (p. 9) two sp hybrid atomic orbitals disposed at 180° to each other digonal hybridisation) are employed. In each case the s orbital is always involved as it is the one of lowest energy level. 

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