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
Aug 16, 2017

The lowest-energy d-d transition band would mean that the ligand field splitting energy for the octahedral complex, Delta_o, is small.

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 pi acceptors, sigma donors, and pi donors... You may also want to read the this answer for a brief review of crystal field theory vs. ligand field theory.

Inorganic Chemistry, Miessler et al., pg. 371Inorganic Chemistry, Miessler et al., pg. 371

  • 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 pi acceptors promotes low spin, having a lot of pi donors promotes high spin, and having a lot of sigma donors promotes low spin.

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.

A)

["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.

B)

["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.

C)

["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.

D)

["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 Delta_o than A and B, but smaller than C.

Overall:

color(blue)barul(|stackrel(" ")(" "Delta_o(A) < Delta_o(B) < Delta_o(D) < Delta_o(C)" ")|)