Electron counting common ligands

Allyl ligands have features common to metal-alkyl and metal-olefin complexes, and can act as three-electron donor ligands according to the electron counting scheme where total covalency is assumed. The metal ion in these complexes interacts with all three carbon atoms of the allyl functionality in an equivalent manner. 

Table 1.1 Electron counting for commonly encountered ligands...

In practice, specific cluster metal framework geometries are associated with particular electron counts and the number of formal metal-metal bonds present. It is often simplest to analyze structures in terms of these electron counts. Some of the possible metal framework geometries associated with the more common electron counts are illustrated in Fig. 17, and these metal arrays, along with the mode of coordination of the alkyne-substituted ligands, will be used in Section IV,C to order the discussion of the structural types. 

In Table 4 we see some of the common ligands and their electron counts on the ionic and covalent models. In the former, we dissect an M-X bond into M+ and X and in the latter... 

Table 4 Common ligands and their electron counts...

A more recent addition to the half-sandwich chemistry of ruthenium is given by a number of complexes where the central metal obeys a 16 valence electron count. These coordinatively unsaturated (see Coordinative Saturation Unsaturation) metal centers are widely invoked as intermediates or transition states in dissociative substitution processes or in catalytic transformations at transition metal centers. Such complexes are not, however, easily isolated. The most common way to stabilize such species is by coordinating sterically bulky ligands to the metal, thereby preventing further ligand addition. They can also be stabilized in the form of dimeric complexes. 

Electron counts for common ligands according to both schemes are given in Table 13-1. 

Table 3-1 Electron Counting Schemes for Common Ligands...

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