Question

What is the oxidation numbers of each element in the `[Fe(C-=N)_6]^(2+)` complex?

Answer 1

The Cyanide part must be `-1` as can be seen from other cyanides like `KCN`, where it is definitely `K^+ andCN^-`

Explanation:

The 6 `CN^-`'s are partly 'compensated' for by `Fe^(2+)` to get an overall charge of `-4`.

Extra :
The ion is called 'hexacyano-ferrate(II)' There also exists an ion called 'hexacyano-ferrate(III)', with an overall charge of `-3`.

Answer 2

Remove the cyanide ligands sequentially, and you get to `Fe^(2+)`, the ferrous ion.

Explanation:

You have the coordination complex `[Fe(C-=N)_6]^(4-)`. We remove the cyanide (`6xx""^(-):C-=N:`) ligands, and we get to `Fe^(2+)`, the ferrous ion, `Fe(II)`. All I have done here is PRESERVE both MASS and CHARGE.

As regards the oxidation numbers of `C` and `N`, I KNOW that the sum of the oxidation numbers must be `-1`, which is the charge on the ion.

Around nitrogen, there are 5 valence electrons (2 from its lone pair, and 3 electrons from its contribution to the triple bond); that, with 2 inner core electrons, and 7 nuclear protons, means its charge is neutral. Thus nitrogen has a FORMAL oxidation number of 0 in the cyanide anion. Now of course, the sum of the oxidation numbers must equal the charge on the ion; so the oxidation number of `C` in cyanide is `-I`.

To do it the long way, there are 3 electrons around `C` from the triple bond, 2 outer valence electrons as the lone pair, and 2 inner core electrons. Thus 7 electrons in total, and `C` has a formal charge of `-1`, and formal oxidation number of `-I`.

Note (i) that `Fe(II)` salts are still the antidote to cyanide poisoning. In labs where there is extensive usage of cyanide, and should cyanide poisoning occur, there would be Mohr's salt available to mix up if some poor punter ingested cyanide thru mouth or hands.

Note (ii) that some common laboratory solvents can pass thru skin, along with whatever salts are dissolved in the solvent. `"Dimethyl sulfoxide"` is very commonly used as a solvent for `"potassium cyanide"`; the solvent passes thru skin very easily along with its solute. `"DMSO"` solutions of `"potassium cyanide"` are treated with respect.