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

#A)# #"FeCl"_3#
#B)# #"CoCl"_3("py")_3#
#C)# #"RuCl"_2#
#D)# #"NiCl"_2("PPh"_3)_2#

1 Answer
Truong-Son N.
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)# #"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#.

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