Ligand contributions, electron counts

For main group elements the number of framework electrons contributed is equal to (t + a — 2) where v is the number of valence shell electrons of that element, and x is the number of electrons from ligands, eg, for Ff, x = and for Lewis bases, x = 2. Examples of 2n + 2 electron count boranes and heteroboranes, and the number of framework electrons contributed by their skeletal atoms, ate given in Table 1. 

Many organometallic complexes are clusters involving multiple metals that feature metal-metal bonds. The electrons in Me-Me bonds are counted by contributing one electron to each metal connected. Bridging ligands contribute one-half of their electrons to each metal center. Some simple examples in Figure 1.9 illustrate application of the rules. 

At this level of approximation, each bond between the metal and a ligand is considered to be a two-electron sigma bond. For the purposes of electron counting (as in the inert-gas rule), this is normally adequate. The number of such bonds is the coordination number. Each single, simple ligand contributes unity to this number. Thus, a square planar coordination implies a coordination number of 4. An octahedron implies a value of 6. The typical values are also included in Table 4.4. 

When other ligands are present, it is normal in 18-electron counting to assume covalent rather than ionic bonding. In Mn(CO)5X, where X = H, Cl or CH3, Mn and X therefore contribute one electron each to the... 

Similarly, cyclopentadienyl anion can act as a a ligand (Tl1), an allyl ligand (rj3), or, most usually, as a cyclopentadienyl ligand (ri5). The distinction is very important for electron counting as these three different situations contribute 2,4, or 6 electrons, respectively, to the complex. 

Table 3 shows how the first-row carbonyls mostly follow the 18e rule. Each metal contributes the same number of electrons as its group number, and each CO contributes 2e for its lone pair, n Back bonding makes no difference to the electron count for the metal. In the free atom it had one AO for each pair of An electrons it uses for back bonding in the complex it stiU has one filled MO, but now delocalized over metal and ligands. 

For (Tf) -C5H5)Fe(CO)2Cl, an iron atom has 8 electrons beyond its noble gas core. r) -C5H5 is now considered as if it were a neutral ligand (a 5-electron tt system), in which case it would contribute 5 electrons. CO is a 2-electron donor and Cl (counted as if it were a neutral species) is a 1-electron donor. The electron count is... 

In terms of electron counting, the alkyl ligand may be considered a 2-electron donor CR3 (method A) or a 1-electron donor -CR3 (method B). Significant ionic contribution to the bonding may occur in complexes of highly electropositive elements, such as the alkali metals and alkaline earths. 

To use either electron-counting procedure, it is necessary to know how many electrons each ligand in a complex donates to the metal. Table 15.1 gives electron contributions for a variety of ligands for both the neutral atom and oxidation state... 

Another method for determining total electron count treats odd-electron donor ligands (alkyl, Cp, etc.) as anionic, even-electron donors. The number of electrons contributed by the metal is calculated from the metal s oxidation state. Both methods give the same answer for total electron count. 

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