Question

How do you name [Co(en)2CO3]Br?

Answer 1

Assuming you mean...

`["Co"("en")_2"CO"_3]"Br"`,

this is a carbonatobis(ethylenediamine)cobalt(III) bromide isomer (either `Delta` or `Lambda`), because...

• `"en"`, ethylenediamine, is a neutral ligand. The complex name warrants a new kind of prefix, bis, to indicate that there are two of them. If there were three, one would use tris.
• `"CO"_3^(2-)` is the carbonato ligand.
• `"Br"^(-)` in the outer coordination sphere implies that the inner coordination sphere is a `+1` charge overall.
• Thus, the oxidation state of cobalt is `+"III"`...

That gives `overbrace((+3) + 2(0) + (-2))^("inner coordination sphere") + overbrace((-1))^("Br"^(-)) = 0`, for a neutral complex overall. Additionally, the "carbonato" comes before the "bis(ethylenediamine)" alphabetically. The bis is ignored when considering alphabetical order of ligand names. You may want to look at this answer for other examples.

There are, however, two isomers that you should be aware of... the `Delta` and `Lambda` isomers. I recommend you keep reading to see what that means...

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There are two isomers for this complex:

There is not a third, where the carbonato is "bonded" trans, because it is a bidentate ligand. It would be too strained to bond across a `180^@` span.

• `Delta` means that of two identical bidentate ligands, with one of them as a rear fin, the other is a right-handed fin.

• `Lambda` means that of two identical bidentate ligands, with one of them as a rear fin, the other is a left-handed fin.

These are optical isomers, and are non-superimposable mirror images of each other, basically a complicated enantiomer.