Complex ions and co-ordination number problem

[alext87]

The definition of co-ordination of a complex ion is the number of atoms datively bonded to the central ion. It there a situation where one atom in a ligand can donate two lone pairs of electrons to the central ion?

[Tartaglia]

Alkoxide anions can act as 4e donors. The second pair is donated through a pi interaction rather than a sigma interaction. There are plenty of others such as the NR group which can also act as a 4e donor.

 

Neutral ligands such as NO and CR act as 3e donors, but if you were to postulate that they were negatively charged then you could look upon them as 4e donors. It is a matter of interpretation of the metals oxidation state

 

4e electron donating alkyne groups are pretty common too - I've even published some myself. These however donate sideways through the pi CC orbitals and so both carbons are involved ( perhaps not quite what you want)