Counting the electrons of a coordination complex

[CrimpJiggler]

Heres the Monsanto process:

I can't figure out how to determine the electron count for these rhodium complexes. Starting with the one at the top. Heres how I count its electrons: rhodium is a group 9 element so it has 2s electrons and 7d electrons which means it has 9 out of 18 valence electrons to begin with. The 2 CO ligands add an extra 4 electrons, bringing the total to 13. I'm a bit confused about how the iodide ligands work, do they also donate 2 electrons each? If so, that brings the electron count to 17. The ion has a charge of -1, bringing the total electron count to 18. How can this be a coordinatively unsaturated complex? If you add 2 more ligands to it then you end up with an electron count of 22.

 

[jbsteele]

Your process for counting the electrons in the rhodium complex is correct. The iodide ligands do indeed donate two electrons each, bringing the total electron count to 17. This is a coordinatively unsaturated complex because it does not have a full octet of electrons around the central atom. In order for the complex to become saturated, two more ligands would need to be added, which would bring the electron count up to 22. The key to understanding why this is an unsaturated complex is that the central atom, rhodium, only has nine valence electrons. The two CO ligands donate four electrons, leaving five electrons to be donated by the other three ligands. With only three additional ligands, there is no way to donate the remaining five electrons, meaning that the complex is coordinatively unsaturated.