Name: 1 CHEM 3111/6170 Second test Topics, metal chemistry, coordination compounds, electronic spectroscopy of transition metal compounds October 29th, 2003 50 minutes to complete Maximum score 76 points 1) Classify all of the following ligands according to their bonding modality as some combination ofσ donor, π acceptor and π donor. (6 points) Phenanthroline σ donor and π acceptor NH 3 σ donor H 2 O σ donor and π donorBr- σ donor and π donor CO σ donor and π acceptor CH 3 - σ donor2) Rank the following groups of ligands according to their ability to produce large values of∆. Put the ligand giving the largest ∆ first. No partial credit. (4 points) a) phen, H 2 O, Cl - phen > H 2 O > Cl - b) CO, NH 3 , I - CO > NH 3 > I - 3) Very briefly explain why most octahedral coordination complexes of Co(III) are diamagnetic, but CoF 6 3- is paramagnetic. (4 points) Most octahedral cobalt complexes are low spin and hence diamagnetic. CoF 6 3- is paramagnetic as it is high spin. The metal is high spin because the fluoride ligands produce a very small ∆ value ( π donors) and B is quite high because the complex is not very covalent.
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4) Very few Ti(II) compounds have been synthesized and studied using optical spectroscopy.
However, dissolution of TiCl2 in molten aluminum chloride is thought to give rise to species
similar to octahedral TiCl64-. The optical spectrum of this melt shows the presence of two
absorption bands at ν1 = 7600 cm-1 and ν2 = 14,500 cm-1.
a) Using one of the attached Tanabe-Sugano diagrams assign these spectroscopic transitions. ( 4points)
ν1 3T2g <-
3T1g
ν2 3T1g <-
3T1g
b) Estimate values for ∆ and B' for TiCl64- (6 points)
ν1/ν2 ~ 1.91. This corresponds to ∆/B ~ 16.7
For ν2 E/B ~ 28.0 if ∆/B ~ 16.7.
This gives B = 518 cm-1
and ∆ = 8648 cm-1
c) B for gas phase free Ti2+ is 718 cm-1. Explain why the free ion value is different from the one
you have just determined. (2 points)
B for the complex is lower than for the free ion as in the complex the electrons are partially
delocalized onto the ligands and this reduces the amount of electron electron repulsion.
5) For each of the following colored compound indicate the primary mechanism that is responsible
for the color of the compound. Your options are A) spin forbidden d-d transition, B) d-dtransition, C) Ligand to metal charge transfer, D) Metal to ligand charge transfer and E) charge
transfer between metals in different oxidation states.
6) NiCl42- is paramagnetic, Ni(CN)42- is diamagnetic and BPh4
- (Ph= pheyl) is also diamagnetic.
Using general chemical principles plus the observed magnetic properties, deduce the shapes of
these three species. (6 points)
NiCl42- Tetrahedral
Ni(CN)42- Square planar
BPh4- Tetrahedral
7) Draw all the possible isomers for a complex of the type Co(en)X2Y2. Indicate which ones are
enantiomers. (8 points)
Cl
Co
Cl
N NH3
NH3 N
+
+
Co N Cl
Cl N
NH3
NH3
+
Co N Cl
NH3 N
Cl
NH3
+
Co N NH3
Cl N
Cl
NH3
The last two are related as enantiomers (they are chiral)
8) For each of the following groups identify one ion that is most likely to form a Jahn-Teller distorted six coordinate complex with the ligand NH3. (6 points)
a) Cu2+, Co3+, Cr 3+, Ti4+
b) Ca2+
, Zn2+, Mn2+, Mn3+
c) Fe
3+
, Co3+
, Cr2+
, Cr 3+
9) Sodium dissolves in dry liquid ammonia to give a blue solution. Write a balanced equationshowing the reaction that gives rise to the blue colored product. (2 points)