Chem 332.3 Fall 2005 Inorganic Chemistry II
Instructor:
Lab Manager:
Course Web Site:
Course Text:
Lecture Hours:
Laboratory Hours:
Dr. Stephen Foley, Office: Thorv 255
Tel: 966-2960; email: stephen.foley@usask.ca
Dr. Pia Wennek, Office: SG 32
Tel: 966-1628; e-mail: pia.wennek@usask.ca
http://www.usask.ca/chemistry/courses/332/
C. E. Housecraft and A. G. Sharpe, Inorganic Chemistry , 2nd Ed. 2005, Pearson/Prentice Hall.
Mon, Weds, and Fri: 11:30-12:20 in Thorv 125
Mon, Tues: 1.30-5.30 in SG 215
Method of Evaluation:
Final Exam (3 hour):
Mid-term Exam:
Assignments (including oral presentation):
Laboratory:
50%
15%
10%
25%
Course Description: An introduction to transition metal chemistry including coordination geometry and stereochemistry, ligand field theory, spectroscopic, magnetic and thermodynamic properties of inorganic compounds, organometallic chemistry and homogeneous catalysis. The laboratory work includes experiments on the preparation and characterization of transition metal compounds.
Topics to be covered (Chapter references are to Housecraft et al.):
Molecular Symmetry (Chapter 3)
Coordination Chemistry
Molecular structure and bonding (Chapters 1, 20) d-block metal complexes (Chapters 19, 20)
Electronic spectra of complexes (Chapter 20)
NMR spectroscopy in inorganic chemistry (Chapter 2)
Descriptive Chemistry of the Transition Metal Elements (Chapters 21-22)
Organometallic Chemistry of the d-block (Chapter 23)
Types of ligands. Bonding. Electron counting. Reaction mechanisms.
Catalysis (Chapter 26 )
General Principles. Homogeneous catalysis.
Molecular Symmetry
Why is symmetry important?
- Symmetry is important for all kind of spectroscopy!
- Usage of symmetry simplifies theoretical calculations
- Many properties are symmetry related
Application of a symmetry operation must not alter the molecule or its properties.
A symmetry operation (SO) is a movement of a body such that, after the movement has been carried out, every point of the body is coincident with an equivalent point (“before and after are the same”).
Effect of a SO is to take the body into an equivalent configuration.
Each symmetry operation is related to a symmetry element.
A symmetry element (SE) is a geometrical entity (a line, a plane, a point) with respect to which one or more SOs may be carried out.
1
Symmetry element n-Fold symmetry axis
Mirror plane
Center of inversion n-Fold symmetry axis of improper rotation
Symmetry operation
Identity
Rotation by 2 π /n
Reflection
Inversion
Rotation by 2 π /n followed by reflection perpendicular to rotation axis
Symbol
E i
C n
σ
S n
Note that E is a special case of C n
E = C
1
, S
1
= σ , S
2
= i.
, and that i and σ are special cases of S n
:
2
SO: Proper rotation by 2 π /n or 360°/n SE: Axis of rotation C n
- the molecule appears unchanged after a rotation by 360°/n. principal rotation axis
- in cases in which more than one rotational axis is present, the one of highest order
4
SO: Reflection SE: Mirror planes σ
- reflection of all atoms through a plane passing through the molecule.
C
6
σ v vertical plane
- plane that contains the principal axis
σ d dihedral plane
- plane that contains the principal axis and bisect the angle formed between adjacent
C
2 axes.
σ h horizontal plane
- plane perpendicular to the principal axis
*
5
1

2

SO: Inversion SE: Center of inversion i
- if it is possible to move in a straight line from every atom of a molecule through a single point to an identical atom at the same distance on the other side of the center, the molecule has a center of inversion.
14
SO: Improper rotation by 2 π /n SE: n-Fold symmetry axis S n
- the combination of a rotation by 360°/n followed by reflection across a plane perpendicular to the rotation axis.
S
4
S
1
= σ S
2
= i
15
SO: Identity
- the identity operation is denoted by E.
- it leaves the whole molecule unchanged.
- every molecule has at least this operation.
- it is included for mathematical completeness.
17
D
3 h
D
4 h
D
∞ h
T d
O h
Point group
C
1
C
2
C s
C
2v
C
3v
C
∞ v
D
2 h symmetry element
E
E , C
2
E, σ
E , C
2
, σ v
, σ v
E , C
3
, 3 σ v
E , C
∞
, ∞ σ v
E , C
2
(x, y, z), σ (xy,yz,zx), i
E , C
3
, 3 C
2
, 3 σ v
, σ h
, S
3
E , C
4
, 2 C
2
’, 2 C
2
’’, i , S
4
, σ h
, 2 σ v
, 2 σ d
E , C
∞
, ∞ C
2
, ∞ σ v
, i, S
∞
E , 4 C
3
, 3 C
2
, 6 σ d
, 3 S
4
E , 3 C
4
, 4 C
3
, 6 C
2
, i , 4 S
6
, 3 S
4
, 3 σ h
, 6 σ d shape examples
SiBrClFI
H
2
O
2
NHF
2
H
2
O, SO
2
Cl
2
NH
3
, PCl
3
, POCl
3
CO, HCl, OCS
N
2
O
4
, B
2
H
6
BF
3
, PCl
5
XeF
4
, trans -MA
4
B
2
H
2
, CO
2
, C
2
H
2
CH
4
, SiCl
4
SF
6
18
3

Decision tree for identifying a molecular point group
The Point Groups of Molecules
Determination of the point group of a molecule: method 1
1. Locate the principal axis of symmetry , this determines the index n.
2. Locate secondary axes C
2
, C
3
, C
4
, or C
5 at an angle to the principal axis.
This determines the primary symbol C , D , T , O or I .
none → C , nC
2
→ D n
, 4 C
3 and C
2
→ T , 3 C
4 and C
2
, C
3
→ O , 6 C
5 and C
2
, C
3
3. Identify the mirror planes and their positions with respect to the principal axis.
→ I.
This determines the index h, v or d.
or method 2 : Use the decision tree . groups of low symmetry: C
1
, C s
, C groups of n-folded rotation axis: C n dihedral groups: D n
, D nh
, D nd i groups with very high symmetry: T d
, C nv
, O
, C h
, I h nh
Point group Number of SOs
C n
C nv
, C nh
, D n
D nd
, D nh
T d
O h
I h n
2 x n
2 x 2 x n
24
48
120
22
Applications of Symmetry
Polar molecules
A polar molecule is a molecule with a permanent dipole moment.
Dipole moment: µ = δ x d (unit = debye (D)).
Certain symmetry elements rule out a permanent electric dipole moment in molecules:
A molecule cannot be polar if it has a center of inversion.
A molecule cannot have an electric dipole moment perpendicular to any mirror plane.
A molecule cannot have an electric dipole moment perpendicular to any axis of rotation.
I
Symmetry forbidden:
(with respect to possible dipole moments)
C i
S n
C nh
D n
D nh
D nd
( C n
( C n
T
O h d h
( C
( C n n
(4 C
3
( i , C
4
( i , C
5
+ σ h
)
+ n C
2
)
+n C
+ n C
2
2
+ σ h
)
)
+ 3 C
2
)
+ 4 C
2
+ 5 C
2
, and σ h
, and σ h
)
)
Symmetry allowed:
C
1
C s
C n
C nv
24
4

Applications of symmetry
Chiral molecules
A chiral molecule is a molecule that cannot superimposed on its own mirror image.
Chiral molecules are optically active (they can rotate the plane of polarized light).
In organic chemistry chirality is based upon asymmetric carbon atoms
(nonsuperimposable mirror image).
Inorganic molecules may be optically active based on asymmetric atoms (N, P, or S), but most compounds are chiral because of the overall molecular symmetry.
A molecule is not chiral if it possesses an improper rotation axis S n
(recall that S
1
= σ and S
2
= i)
25
Chiral
C
1
C n
D n
(asymmetric)
(dissymmetric)
(dissymmetric)
Achiral (identifying symmetry element)
C s
C i
D nh
(plane of symmetry)
(center of symmetry)
(plane of symmetry)
(plane of symmetry) D nd
S n
T d
O h
I h
C nv
C nh
(improper axis)
(plane of symmetry)
(center and plane of symmetry)
(center and plane of symmetry)
(plane of symmetry)
(center and plane of symmetry)
26
5

IR Spectroscopy
The four normal modes for CO
2
.
A vibration is infrared active, when there is a change of the dipole moment.
Therefore the symmetric stretching mode is IR inactive.
Information from the symmetry of normal modes square-planar complexes
Both have bands in the Pd-Cl stretching region between 200-400 cm -1 .
33
IR Spectroscopy
IR Spectroscopy
(Excitation of molecular vibrations by infrared radiation).
Number of independent motions of an N atomic molecule is 3N.
Number of vibrational modes of a molecule is
3N-6 (or 3N-5 if linear) = number of the normal modes.
Wavenumber: ν = cm -1 region: 4000-40 cm -1 stretching bending stretching
32
6

Chapter 19 (Housecroft)
Coordination Chemistry
Coordination Compound ML n
Central metal atom M surrounded by a set of ligands L. n is the number of ligands and corresponds to the coordination number n of the center atom.
Combination of a Lewis acid (center metal atom) and a Lewis base (ligand).
Ligand May be charged or uncharged, and may consist of one or more atoms.
Donor atom
Acceptor atom
The atom in the ligand which forms the bond to the centre atom.
Metal atom or ion, which is the Lewis acid.
.
Examples:
[Co(NH
3
)
6
]Cl
3
Ni(CO)
[PtCl
4
]
4
2-
Trichlorohexaamminecobalt(III)
Tetracarbonylnickel
Tetrachloroplatinate(II)
42
7

Coordination Chemistry
Organometallic Chemistry
One class of coordination compounds which involves metal carbon bonds.
Bioinorganic Chemistry
Coordination compounds which are present in living organism.
Alfred Werner (Swiss chemist 1866-1919)
Nobel prize in chemistry 1913
"in recognition of his work on the linkage of atoms in molecules by which he has thrown new light on earlier investigations and opened up new fields of research especially in inorganic chemistry"
First Nobel prize in inorganic chemistry.
43
8

Coordination Chemistry
The problem:
4 coordination compounds of cobalt (III) chloride with ammonia has been discovered and named according to their colours:
CoCl
3
·6NH
3
CoCl
3
·5NH
3
CoCl
3
·4NH
3
CoCl
3
·4NH
3 yellow purple green violet
Luteo complex
Purpureo complex praseo complex violeo complex
Addition of Ag + :
CoCl
3
·6NH
3
CoCl
3
·5NH
3
CoCl
3
·4NH
3
+ excess Ag
+ excess Ag
+ excess Ag
+
+
+
→
→
→
3AgCl (s)
2AgCl (s)
1AgCl (s)
Werner’s conclusion:
Compound Co(NH
3
)
5
Cl
3 is derived from compound Co(NH
3
)
6
Cl
3 by loss of one ammonia. With this loss a change in function of one chloride ion occurs (one chlorine no longer acts as an ion).
Werner’s formulation of the compounds:
[Co(NH
3
)
6
]Cl
3
, [Co(NH
3
)
5
Cl]Cl
2
, [Co(NH
3
)
Difference in the conductivity.
4
Cl
2
]Cl,
50
Coordination Chemistry
Werner assigned the correct geometric structure to many coordination compounds long before any direct methods were able to determine structures.
(like X-ray diffraction, nuclear magnetic resonance)
He used patterns of reactions and isomerism.
Example: The structure of [Co(NH
3
)
6
] 3+
His postulation: Symmetrical arrangement of the ammonia molecules around the center cobalt atom.
The numbers of isomers found was equal to that expected for an octahedral complex.
51
Coordination Chemistry
Geometrical isomerism in six-coordination
[MX
4
Y
2
]
[MX
3
Y
3
] mer (for meridional) fac for facial 52
9

Coordination Chemistry
Central metal atom could have coordination numbers from 2 to 12.
Three factors determine the coordination number of a complex:
1. The size of the central atom or ion.
2. The steric interactions between the ligands.
3. Electronic interactions.
Higher coordination numbers in the complexes of atoms and ions in period 5 and 6.
Higher coordination numbers on the left side of a row of the d -block.
Especially when the metal ion has only few d electrons, so that it can accept more electrons from Lewis bases.
e.g. [Mo(CN)
8
] 4octacyanomolybdate(IV)
Low coordination numbers with bulky ligands.
Lower coordination numbers are found on the right side of the d -block.
Especially when the metal ion has a high number of d electrons.
e.g. [PtCl
4
] 2tetrachloroplatinate(IV)
57
Coordination Chemistry
Ligands monodentate ligands - one point of attachment to the metal atom
Br , Cl , OH H
2
O, NH
3
, CO, polydentate ligands - more than one point of attachment
(CH
3
COCHCOCH
3
)- (acac), H
2
NCH
2
CH
2
NH
2
(en) , C
2
O
4
2(ox) ambidentate ligands - different potential donor atoms
NO
2
, SCN chelating ligand - polydentate ligands can produce a chelate, a complex in which the ligand forms a ring that includes the metal atom.
five- and six-membered chelate rings are especially stable.
59 acetylacetonato
Coordination Chemistry
Ligands
The ligand ethylendiamintetraacetato can attach the metal at six points and can form five five-membered rings. complexometric titration of Ca 2+ or Zn 2+
Bite angle
L-M-L angle in the chelate a bidentate ligand with a small bite angle can result in distortions from standard structures.
distortion from octahedral to trigonal-prismatic
60
10

Coordination Chemistry
Optical isomers
Chirality and optical isomerism in six-coordination chiral – not superimposable on its own mirror image enantiomers – two mirror-image isomers enantiomers of cis -[CoCl
2
(en)
2
] trans -[CoCl
2
(en)
2
]
Remember the criterion for chirality is the absence of an axis of improper rotation, which includes the absence of a mirror plane of an inversion center i = S
2
.
σ = S
1 and the absence
61 right-hand screw left-hand screw
Coordination Chemistry
View along a three-folded rotation axis to determine the handedness of the helix formed by the ligands.
62
Chelate Effect
The chelate effect or chelation is one of the most important ligand effects in transition metal coordination chemistry. Since most metal-ligand bonds are relatively weak compared to C-C bonds, M-L bonds can often be broken rather easily, leading to dissociation of the ligand from the metal. Consider the two metal ligand complexes shown below:
L
M L
L
M
+
L
L
M L
L
M
L
x
“etax ” was originally developed to indicate how many carbons of a π -system were coordinated to a metal center. Hapticity is another word used to describe the bonding mode of a ligand to a metal center.
x
“mux ” is the nomenclature used to indicate the presence of a bridging ligand between two or more metal centers.
The x refers to the number of metal centers being bridged by the ligand. Usually most authors omit x = 2 and just use
µ to indicate that the ligand is bridging the simplest case of two metals.
¾
¾
¾
¾
¾
11

67
Bonding of Coordination Compounds
Valence Bond Theory of Complexes
1930 developed by Linus Pauling and J.C.Slater
Covalent approach: the bonded atoms share an electron pair.
The two electrons come from the same atom, the ligand L.
Metal atom or ion is acting as a Lewis acid
Ligand is acting as a Lewis base
Use of hybridization of metal s, p, and d valence orbitals
This model makes it possible:
- to predict composition and stability
- to interpret the structure and magnetic properties of the complexes
This model cannot explain the colours of complexes
(spectroscopic properties).
69
12

13

14