Metal-Ligand n Bonding

Each local metal-ligand interaction in a complex might include a contribution from K bonding. We now remove the restriction of only local 7 bonding adopted above. 

We now incorporate the bonding and antibonding t2g molecular orbitals of Fig. 6-10 into the energy diagram of Fig. 6-9. So as not to obscure the important issues 

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Figure 6-11. Metal - ligand n bonding in symmetry. The boxed orbitals are filled with electrons - nationally from the group of six ligands.

In their metal complexes, bonding of either species to the metal atom is via a ligand - > metal a donor bond and a metal - >ligand n bond, enabling back donation of electron density to the n orbitals of the C-C multiple bond system to take place. Vinylidene is one of the best 7t-acceptors known, and is exceeded only by S02 and CS in this respect the relationship between phenylvinylidene and other common ligands has been determined (18) from the CO force constants exhibited by a series of Mn(CO)2(q-C5H5) complexes, which increase in this order ... 

Substitution Which Increases the Ionic Character of the Metal-Ligand n-Bond... 

In the case of the octahedral metal carbonyls Cr(CO)6 and Mo(CO)6, pairs of intense bands at 35,800 and 44,500 cm-1 for the former and 35,000 and 43,000 cm-1 for the latter have been plausibly assigned to transitions from the bonding (mainly metal) to the antibonding (mainly ligand) components of the metal—ligand n bonding interactions.21... 

Just as in the olefin disproportionation reaction (Fig. 10), it is necessary to put electron density into the hz orbital. This destroys the carbon-carbon n bonding, the metal-ligand n bonding, and creates the new carbon-carbon a bond. However, instead of taking electrons from the bi orbital of the complex, let us now take it from a filled d orbital concentrated on the metal of fli symmetry. 

Figure 8.5.1. Dependence of n(k2/k i) in replacement of MeOH of [Co (PP)(MeO)(MeOH)] (5x10 M) by various pyridine type ligands (2xl0 - M) on their pK values at 25 °C. The descending branch is inferred to correspond to the decrease in metal —> ligand n bonding and the ascending branch to the increase of a bonding. " ...

Finally, it should be mentioned that the replacement rates of coordinated methanol, supposedly related to the metal —> ligand n bonding, are associated with negative entropies of activation, as one would expect because of the increased order in the reaction transition state due to it bonding (and solvent association). 

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