Download Solutions to Problem Set #9

Chemistry 1311
Problem Set 9
From text (RC) Chapter 19: problems 5, 6 (compounds are [Co(en)3]Cl3 and
[Co(NH3)3Cl3]), 7, 10, 14, 15-17, 23 (contrary to what the problem indicates the more
common abbreviation for this tridentate ligand is actually dien), 26, 31
19.5 In addition to cis- and trans-[Pt(NH3)2Cl2], the other compound of the same
empirical formula is [Pt(NH3)4][PtCl4]. It is a structural isomer of the other two
since it has different atom connectivities.
19.6 As a tris-chelate complex the cation [Co(en)3]3+ exists in non-superimposable
mirror-image forms. [Co(NH3)3Cl3] has two diastereoisomeric forms meridonal and
facial.
19.7
H2
N
Cl
H2
N
1+
Co
N
H2
Cl
N
H2
NH2
H2N
H 2N
1+
Cl
Cl
Co
Co
Cl
NH2
1+
H 2N
Cl
NH2
NH2
H 2N
19.10 [Cr(NH3)6]Cl3 [Co(H2NCH2CH2NH2)2(OH2)(NCS)]NO3 K2[Ni(CN)4]
[Co(H 2NCH2CH2NH2)3]I3
19.14 [Fe(CN)6]3- will be low spin because cyanide is a strong field ligand which in itself
is enough, but this is also a trivalent metal ion, which also serves to increase the
crystal field splitting (relative to a divalent ion). [FeCl4]- will be high spin because
the crystal field splitting for tetrahedral complexes is always small and they are
therefore always high spin. It is worth noting that chloride ion is a weak field
ligand and that the complex contains Fe(II) which results in a smaller crystal field
splitting than would exist for a higher oxidation state.
19.15 [Co(NH3)6]3+ has the largest crystal field splitting because it contains a +3 ion; this
complex is low-spin d6. The analogous +2 ion has a value that is roughly one half
as large, which is predicted for a difference in oxidation state of one; it is high-spin
d7. The stereochemistry of [Co(NH3)4]2+ is not given, but assuming that it is
tetrahedral the crystal field splitting for tetrahedral complexes is only about 4/9
that of octahedral complexes of similar ligands. If tetrahedral it is high-spin.
19.16 [CrF6]3- and [Cr(OH2)6]3+ both contain Cr3+. The difference in their crystal field
splitting is due to the difference in ligand field strength for fluoride (smaller) vs
water (larger). [CrF6]2- contains Cr4+; the increase in crystal field splitting is due to
the increase charge on the metal. [Cr(CN)6]3- contains Cr3+; the large value for the
crystal field splitting reflects the large ligand field strength of the cyanide ligand.
19.17 (a) Both [MnF6]2- and [ReF6]2- contain metal ions in the +4 oxidation state. The
crystal field splitting will be larger for the latter ion because it is in the third
transition series. Metal-ligand interactions increase in strength in a family due to
increased orbital overlap between metal and ligand orbitals and increased
covalency in the bond.
(b) [Fe(CN)6]4- and [Fe(CN)6]3- contain iron in the +2 and +3 oxidation state,
respectively. The latter ion will have the larger crystal field due to the increased
ion charge.
19.23 [Ni(OH2)6]2+ + 2 dien = [Ni(dien)2]2+ + 6 H2O. This reaction will be strongly favored
because of the increase in entropy resulting from the net gain of four free
molecules (chelate effect). Note that the problem did not state whether a 1:1 or
1:2 complex was formed. The latter was assumed in writing the balanced
reaction; however, the chelate effect would operate if only a 1:1 complex was
formed. There is also a very favorable enthalpy change for both reactions.
19.25 The difference in stoichiometry is a result of the difference in size of the ligands.
The tricyclohexylphosphine is so large that only one can be accommodated.
Probably [FeL3Cl3] would be formed with a sufficiently small phosphorus ligand,
perhaps with trimethylphosphine. The difference in sizes is reflected in the spacefilling models of these ligands shown below.
P(CH3)3
P(C6H5)3
P(C6H11)3
19.26 Because [Ni(PPh3)2Cl2], a d8 four-coordinate complex, is paramagnetic it must be
tetrahedral (recall that all square-planar d8 complexes are low-spin and
diamagnetic) and there is only one stereoisomer. Because [Pd(PPh3)2Cl2] is
diamagnetic and also contains a d8 ion it must be square-planar. Therefore there
are two stereoisomers (diastereoisomers also known as geometrical isomers) cisand trans-.
19.31 The three compounds are [Cr(OH2)6]Cl3, hexaquachromium(III) chloride;
[Cr(OH2)5Cl]Cl2, pentaaquachlorochromium(III) chloride; [Cr(OH2)4Cl2]Cl,
tetraaquadichlorochromium(III) chloride.
1. Draw structures for the following molecules or ions and identify the potential donor
atoms in each. P(CH3)3, H2NCH2CH2SCH2CH2NH2, SO32-, NCS-, oxalate ion (C2O42-),
glycinate (H2NCH2CO2-).
P
H3C
CH3
O
O
C
C
H2N
CH3
O
O
NH2
S
-
S
- O
O
-
N
O
C
S
-
O
H2 N
-
O -
2. Covalent interactions of ligands with transition metal ions can have dramatic effects
on the energy separation of the two groups of d orbitals in an octahedral complex. All
ligands bond more or less strongly through σ interactions but it is the presence, and
type, of π interaction that differentiates ligands into weak (π donor), intermediate (often
no π interaction), and strong field (π acceptor) types. Specify a ligand of each type and
for the π donor and π acceptor ligands indicate what orbital(s) on the ligand are used for
π bonding to the metal.
Ligand
Orbital used for π bonding
σ only
H- or NH3
Not applicable
π acceptor
CO or PR3
π*, 3d
π donor
halide
np
3. Draw structures of the diastereoisomers that can exist for the complex ion
[Co(H2NCH2CH2NH2)(NH3)2Cl2]+ ? Which of these diastereoisomers is enantiomeric?
N
N
NH3
Co
NH3
Cl
Cl
N
Cl
N
Co
Cl
NH3
N
NH3
N
NH3
Co
Cl
NH3
Cl
4. Draw structures for all of the diastereoisomers of [Co(dien)(Cl)(NO2)2]? dien is
H2NCH2CH2NHCH2CH2NH2. Which of these diastereoisomers is enantiomeric?
N
N
O2N
N
NO2
Cl
N
N
Cl
Cl
Co
N
NO2
N
N
N
NO2
Cl
Cl
N
NO2
NO2
N
N
NO2
Cl
Cl
NO2
5. Draw structures for all stereoisomers (diastereoisomers and enantiomeric forms) of
[Co(H2NCH2CH2O)3]?
N
O
N
Co
O
N
N
N
O
O
Co
N
N
N
O
O
N
Co
O
O
N
N
O
O
Co
O
N
O
6. The tetradentate ligand H2NCH2CH2NHCH2CH2NHCH2CH2NH2 forms six-coordinate
complexes with Co(III) having the composition [CoLX2]+ where X is a mondentate ligand.
Draw structures of all possible diastereoisomers and indicate which are enantiomeric.
trans
α-cis
β-cis