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
Inorganic Chemistry, Miessler et al., pg. 371
bbpi acceptors interact in a backbonding interaction, where the metalt_(2g) orbitals donate electron density back into the ligand's antibonding orbitals. As a result, thet_(2g) orbitals are lowered in energy (because repulsions are lessened), increasingDelta_o .bbpi donors donate electron density into thet_(2g) orbitals (which are the triply-degenerate,pi -compatible orbitals), destabilizing those orbitals and slightly decreasingDelta_o .bbsigma donors donate electron density into the metalsigma -compatible orbitals, raising the energy of the metal antibonding orbitals, thee_g^"*" (in crystal field theory they may be labeled simplye_g ), thus increasingDelta_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 abbpi donor."NH"_3 is a strong-field ligand, because it is abbsigma donor.Since
A contains a weak-field ligand, it is expected to have aDelta_o that is the smallest amongA-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), soDelta_o for this complex is larger than forA .
["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 largeDelta_o . However,"CN"^(-) , cyanide, is both api acceptor andsigma donor, making it VERY strong-field.Hence, this has the largest
Delta_o amongA-C .
["Co"color(red)("NH"_3)("NH"_3)_5]^(3+) , or hexamminecobalt(III) (I wrote the formula like that on purpose), has all sixsigma donors, and so it has a largeDelta_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)" ")|)