Anionic ligands in coordination complexes

[CrimpJiggler]

I read that all ligands are Lewis bases but I then read that the number of anionic ligands attached to a metal will determine its oxidation state. Which is it? Let's use chlorine as an example. Before this ligand binds to the metal is it a Cl^- ion or a neutral Cl atom? If its the former I can see how its a Lewis base because it provides both electrons for the bond. Conversely I don't see how it could "raise" the oxidation state of the metal if its actually bonded as a Lewis base. If its the latter (if its a neutral Cl atom) then I can see how it raises the oxidation state of the metal by 1 but I don't see how its a Lewis base.

So to summarise the question, what are anionic ligands? Are they anions from the start or do they become anions by taking an electron from the metal?

 

[DGJNAc]

Anionic ligands are ligands that would dissociate from the metal as an anion, and are almost always anions from the start as well, neutral ligands dissociate as a neutral species.

You can use the number of anionic ligands and overall charge of a complex to determine the oxidation state of the metal. Note that the number of anionic ligands can still be 0, in which case the oxidation state is the overall charge of the complex.

Note that all ligands are Lewis bases - Lewis basic and anionic are by no means mutually exclusive!