Download Chem 324 Fall 2009 Quiz #3 KEY NAME: KEY

Chem 324 Fall 2009
Quiz #3 KEY
NAME: KEY
Answer all questions on this paper. There are 12 pts total.
1.
State the basic concept behind crystal field theory in one sentence. (2 pts)
Crystal field theory assumes that ligands, as negative point charges, repel electrons in
metal d orbitals to different extents depending on their relative orientations resulting in
splitting of the d orbital energies.
2.
Explain why a d9 ion will undergo a Jahn Teller distortion in an octahedral field. (2 pts)
Partially filled degenerate levels favour a distortion that breaks the degeneracy of the
level and places more electrons in the stabilized level and fewer in the destabilized level.
In this case the eg3 configuration will distort so that two electrons reside in a stabilized
level and one is a destabilized level.
3.
d8 Ni2+ can adopt either a square planar or tetrahedral geometry in 4-coordinate
complexes whereas d8 Pt2+ adopts the square planar geometry without exception.
Rationalize this observation using: [1 pt each]
(a) an electronic argument
The crystal field splitting favours a square planar arrangement over tetrahedral for
d8 but the magnitude of the difference depends on Δ. The magnitude of Δ increases
significantly from the 3d to the 4d/5d rows due to better radial extension of the 4d
and 5d orbitals so this strongly favours square planar for Pt2+
(b) a steric argument
Steric repulsion favours a tetrahedral geometry but Pt2+ is considerably larger than
Ni2+ so this helps minimize steric repulsions between the ligands.
4.
Determine the d orbital splitting to be expected (qualitatively) in a T-shaped crystal
field (i.e. for a square planar arrangement missing one ligand). Take the complex plane to
be the xy plane and make sure you clearly distinguish between the x and y directions. (4
pt) NOTE: represent your answer as a qualitative energy level diagram.
This can be approached either from first principles or by considering it as a perturbation
on the square planar case:
5.
Rank the following complexes in order from lowest to highest (increasing order) Δoct:
(1 pt each)
(a) FeBr2(OH2)4 vs. FeCl2(en)2
Cl and en are stronger field ligands than Br and H2O so FeCl2(en)2 has a larger Δoct
(b) [Fe(bipy)3]3+ vs. [Fe(OH2)6]2+
bipy is a stronger field ligand than H2O and the smaller size of Fe3+ results in an
increase in Δoct so [Fe(bipy)3]3+ has a larger Δoct on both grounds.