Which of these transition metal complexes contain `3d^6` metals?

The idea here is to know what the charges on the ligands are, and deduce the oxidation state of the central metal.

I got `B`.

`A)` `"FeCl"_3` holds three `"Cl"^(-)` (chloro) ligands, so the iron atom has an oxidation state of `+3`. Iron originally had...

`[Ar]3d^6 4s^2`

but removing three valence electrons (which if you recall, come out of the `ns` first), the `+3` state would have...

`ul([Ar]3d^color(red)(5) 4s^0)`

so this is not it.

`B)` `"CoCl"_3("py")_3`, as before, contains three `"Cl"^(-)` (chloro) ligands. You are actually expected to know that pyridine is a neutral ligand, so that means `"Co"` has a `+3` oxidation state.

Cobalt's neutral electron configuration was originally...

`[Ar]3d^7 4s^2`

so losing three electrons for the `+3` state lands it on `ul(color(blue)(3d^6 4s^0))`.

`C)` `"RuCl"_2`, as we can tell at this point, as `"Ru"("II")`, or the `+2` oxidation state. Ruthenium normally has a configuration of...

`[Kr] 4d^7 5s^1`

(which, by the way, is an Aufbau 'exception'.)

If you did mean `3d^6`, `"Ru"` doesn't apply; it has a

`ul([Kr] color(red)(4)d^6 5s^0)`

configuration with a `+2` oxidation state... wrong period in the periodic table!

`D)` `"NiCl"_2("PPh"_3)_2` has two `"Cl"^(-)` (chloro) ligands, and two triphenylphosphine (TPP) ligands. The TPP ligands are neutral, so the oxidation state on nickel is `+2`.

The neutral configuration was...

`[Ar] 3d^8 4s^2`

And so, the `+2` state has...

`ul([Ar] 3d^color(red)(8))`

which is not `3d^6`.