[Kate]

Hello.

My book says the following: "The simplest approach is to say that a metal is in the (0) oxidation state unless it has σ bonds to ligands such as Cl, AcO, or Me that form bonds with shared electrons. Neutral ligands such as Ph₃P that provide two of their own electrons do not affect the oxidation state of the metal."

I know how to determine oxidation states, that's not the problem. My question is, what is the difference in terms of bonding between anionic ligands, such as Cl or Me, and neutral ligands like phosphines? In the latter, are the electrons to form a bond coming exclusively from the ligand (and as such, the oxidation state of the metal is unchanged), while in the former both the ligand and the metal contribute one electron to form a bond? This doesn't make much sense, since the transition metals are usually electron deficient and the anionic ligands have extra electrons they can share, as opposed to neutral ligands.

Anyway, I guess what I'm asking is: when is a bond to a transition metal dative and when is it covalent?

[mjc123]

The approach of the book seems to be to treat ligands as uncharged (any net charge being on the whole complex). The ligands are then Cl·, AcO· and Me·, and they form bonds with M(0) by sharing electrons, as distinct from ligands like Ph₃P: that donate an electron pair to M(0). The former could also be considered as Cl^- etc. donating an electron pair to M⁺, and often the complexes will be made in practice by adding anions to metal cations. You can look at it either way, but not as Cl^- donating to M(0).

[Kate]

Got it. Thanks.