Metal-ligand bonding, steric consequence

In scrutinizing the various proposed reaction sequences in Eq. (26), one may classify the behavior of carbene complexes toward olefins according to four intimately related considerations (a) relative reactivities of various types of olefins (b) the polar nature of the metal-carbene bond (c) the option of prior coordination of olefin to the transition metal, or direct interaction with the carbene carbon and (d) steric factors, including effects arising from ligands on the transition metal as well as substituents on the olefinic and carbene carbons. Information related to these various influences is by no means exhaustive at this point. Consequently, some apparent contradictions exist which seem to cast doubt on the relevance of various model compound studies to conventional catalysis of the metathesis reaction, a process which unfortunately involves species which elude direct structural determination. 

The chemical behaviour of 133 is a result of two factors the high basicity of the metallic center, as a consequence of the presence of the strong donor phosphine and the chloride ligands in the complex, and the large steric hindrance experienced by the triisopropylphosphine groups, which are mutually cis disposed. This mixture allows access to reactive points on the osmium center by activation of Os-P and Os-Cl bonds. 

A very interesting ligand exchange at the metal atoms is observed in compounds of types 35-38. The two metal atoms within the molecule are in competition for the two nitrogen atoms, of which only one is attainable for steric reasons. As a consequence, one aluminum atom becomes electronically unsaturated and has to participate in a three-center two-electron bond (see also Fig. 2). This bonding situation can be rapidly inverted by an exchange of ligands as shown in Eq. (36) for compound 35 (64). 

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