Which of these complexes has the lowest-energy #d-d# transition band in a UV-Vis spectrum?
#a)# #["CoBr"("NH"_3)_5]^(3+)#
#b)# #["Co"("H"_2"O")("NH"_3)_5]^(3+)#
#c)# #["Co"("CN"-kappa""C)("NH"_3)_5]^(2+)#
#d)# #["Co"("NH"_3)_6]^(3+)#
1 Answer
The lowest-energy
That is when the complex has a high-spin configuration, and the complex has few strong-field ligands or many weak-field ligands.
In the spectrochemical series, the ligand field strength of each ligand is:
#overbrace("Br"^(-))^("pi donor")# #"<<"# #overbrace("H"_2"O")^("mostly sigma donor") < overbrace("NH"_3)^"sigma donor"# #"<<"# #overbrace("CN"^(-))^"pi acceptor AND sigma donor"#
And so we have...
#barul(|stackrel(" ")(" "Delta_o(A) < Delta_o(B) < Delta_o(D) < Delta_o(C)" ")|)#
First off, let's define
#bbpi# acceptors interact in a backbonding interaction, where the metal#t_(2g)# orbitals donate electron density back into the ligand's antibonding orbitals. As a result, the#t_(2g)# orbitals are lowered in energy (because repulsions are lessened), increasing#Delta_o# .#bbpi# donors donate electron density into the#t_(2g)# orbitals (which are the triply-degenerate,#pi# -compatible orbitals), destabilizing those orbitals and slightly decreasing#Delta_o# .#bbsigma# donors donate electron density into the metal#sigma# -compatible orbitals, raising the energy of the metal antibonding orbitals, the#e_g^"*"# (in crystal field theory they may be labeled simply#e_g# ), thus increasing#Delta_o# .
In short... having a lot of
You can look at the spectrochemical series to check which ligands are strong-field and which are weak-field.
https://en.wikipedia.org/wiki/Spectrochemical_series
The one different ligand is highlighted in red and shall be considered.
#["Co"color(red)("Br")("NH"_3)_5]^(2+)# , or pentamminebromocobalt(III), contains:
#"Br"^(-)# is a weak-field ligand, because it is a#bbpi# donor.#"NH"_3# is a strong-field ligand, because it is a#bbsigma# donor.Since
#A# contains a weak-field ligand, it is expected to have a#Delta_o# that is the smallest among#A-D# .
#["Co"color(red)("H"_2"O")("NH"_3)_5]^(3+)# , or pentammineaquacobalt(III), likewise has many strong-field ammine ligands, so this is going to be low spin, i.e.#Delta_o# is large.In fact, water is a
#sigma# donor (primarily), so#Delta_o# for this complex is larger than for#A# .
#["Co"color(red)("CN")("NH"_3)_5]^(2+)# , or pentamminecyanocobalt(III), has five#"NH"_3# ligands, so you again know that this is going to have a large#Delta_o# . However,#"CN"^(-)# , cyanide, is both a#pi# acceptor and#sigma# donor, making it VERY strong-field.Hence, this has the largest
#Delta_o# among#A-C# .
#["Co"color(red)("NH"_3)("NH"_3)_5]^(3+)# , or hexamminecobalt(III) (I wrote the formula like that on purpose), has all six#sigma# donors, and so it has a large#Delta_o# , being a low-spin complex.
Comparing, this has a larger
Overall:
#color(blue)barul(|stackrel(" ")(" "Delta_o(A) < Delta_o(B) < Delta_o(D) < Delta_o(C)" ")|)#