Other Important Ligands

Complexes containing direct metal-carbon single, double, and triple bonds have been studied extensively the most important ligands having these types of bonds are summarized in Table 6-1. 

Some of the earliest known organometallic complexes were those having a bonds between main group metal atoms and alkyl groups. Examples include 

Grignard reagents, which have magnesium-alkyl bonds, and alkyl complexes with alkali metals, such as methyllithium. The syntheses and reactions of these main group compounds are typically discussed in detail in organic chemistry texts. 

The first stable transition metal alkyls were synthesized in the first decade of the 20th century many such complexes are now known. Examples of stable transition metal alkyls include Ti(CH3)4, W(CH3)6, and Cr[CH2Si(CH3)3]4. The metal-ligand bonding in these compounds may be viewed as primarily involving covalent sharing of electrons between the metal and the carbon in a a fashion, as shown below. 

In terms of electron counting, the alkyl ligand may be considered the two-electron donor, CR3 (method A), or the one-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. 

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Other important ligands include those based upon the diazafluorene structure <2000JST91>. It has been shown that the coordination behavior of complexes containing the diazafluorene ring system is quite different to that of the parent 2,2 -bipyridyl or 1,10-phenanthroline systems, and evidence for unsymmetrical chelation has been observed <1998POL2463, 1996IC2253>. 

Several other important ligands have direct metal-carbon ct bonds. Examples are given in Table 13-5. In addition, there are many examples of metallacycles, complexes containing metals incorporated into organic rings. The following reaction provides an example of a metallacycle synthesis ... 

The list of ligands capable of bonding to metals through carbon is still in complete. Examples of several other important ligands not yet considered are given in Table 6-3. 

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