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

## A) ${\text{FeCl}}_{3}$ B) "CoCl"_3("py")_3 C) ${\text{RuCl}}_{2}$ D) "NiCl"_2("PPh"_3)_2

Aug 16, 2017

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) ${\text{FeCl}}_{3}$ holds three ${\text{Cl}}^{-}$ (chloro) ligands, so the iron atom has an oxidation state of $+ 3$. Iron originally had...

$\left[A r\right] 3 {d}^{6} 4 {s}^{2}$

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

$\underline{\left[A r\right] 3 {d}^{\textcolor{red}{5}} 4 {s}^{0}}$

so this is not it.

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

Cobalt's neutral electron configuration was originally...

$\left[A r\right] 3 {d}^{7} 4 {s}^{2}$

so losing three electrons for the $+ 3$ state lands it on $\underline{\textcolor{b l u e}{3 {d}^{6} 4 {s}^{0}}}$.

C) ${\text{RuCl}}_{2}$, as we can tell at this point, as "Ru"("II"), or the $+ 2$ oxidation state. Ruthenium normally has a configuration of...

$\left[K r\right] 4 {d}^{7} 5 {s}^{1}$

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

If you did mean $3 {d}^{6}$, $\text{Ru}$ doesn't apply; it has a

$\underline{\left[K r\right] \textcolor{red}{4} {d}^{6} 5 {s}^{0}}$

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

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

The neutral configuration was...

$\left[A r\right] 3 {d}^{8} 4 {s}^{2}$

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

$\underline{\left[A r\right] 3 {d}^{\textcolor{red}{8}}}$

which is not $3 {d}^{6}$.