Question

What is the oxidation number with respect to chromium, and the coordination number for the complex `[Cr(en)_2Cl_2]^(3+)` `("en=ethylenediammine")`?

Answer 1

The coordination number of chromium is 6.

The oxidation number of the metal is `III^+`.

Explanation:

There are 6 nitrogen donors arranged around the metal centre in an octahedron; the `en` ligands are bidentate (2 donors!), and each occupy 2 adjacent sites in the metal's coordination sphere. Since the ammine donors are neutral, and the metal centre is `Cr^(3+)`, the charge on the complex is balanced by three chloride counter ions that are along for the ride and are outside the coordination sphere as per the formula, `[Cr(en)_2(NH_3)_2]^(3+)`.

Note that the complex still has isomers possible: an arrangement where the unidentate ammine donors are trans, versus the isomer where these are `cis`, and (again confusingly) this `cis` isomer will have 2 optical isomers possible. The `cis` versus `trans` isomers are known, and certainly are different compounds, These are classic Werner complexes.

The richness of the coordination geometry is one of the reasons why this complex is chosen as an exemplar. Can you tell me why the ethylenediammine ligands are disposed `cis` and cannot coordinate in a `trans` fashion?