Filling compound effects, analysis structure

The effects of rr-donation from the filled alkyne orbital orthogonal to the n-donating orbital are illustrated by the three structurally characterized Mo(II) diphenylacetylene complexes shown in Figure 2.28. Hoffmann has analyzed the bonding in these complexes. The alkyne ligand in the first complex is adequately described as a two-electron donor, whereas a detailed electronic analysis shows that the alkyne in the porphyrin complex must participate in additional ir-donation to the metal. As a result of this difference in bonding, the acetylenic C-C distance increases across the series of compounds, and the distance from the midpoint of the acetylene to the Mo decreases. 

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