Answer to Question #162716 in Organic Chemistry for Helen

Question

The stability of metal carbonyl complexes falls off markedly as we go to the right of group 10 in the periodic table. For example, Cu complexes only bind CO weakly. Why is this? What oxidation state, of the ones commonly available to copper, would you expect to bind CO most strongly?

Answer 1

In metal carbonyl complexes CO acts as a weak sigma donor and a strong Pi acceptor ligand. In Metal-CO complexes, if a weak sigma donation from CO and a strong pi acceptance to CO from metal takes place then a strong complexes are generally formed. In Cu zero oxidation state i.e the complexes where Cu is in zero oxidation state, weak pi donation from metal to CO taking place. That is why Cu complexes only bind CO weakly.

The most common oxidation number of Cu is +2. Here complexes where Cu is in +2, binding with CO is not expected to be strong.