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Chem 332.3 Fall 2005 Inorganic Chemistry II

Instructor: Dr. Stephen Foley, Office: Thorv 255Tel: 966-2960; email: stephen.foley@usask.ca

Lab Manager: Dr. Pia Wennek, Office: SG 32Tel: 966-1628; e-mail: pia.wennek@usask.ca

Course Web Site: http://www.usask.ca/chemistry/courses/332/Course Text: C. E. Housecraft and A. G. Sharpe, Inorganic Chemistry, 2nd Ed. 2005, Pearson/Prentice Hall.Lecture Hours: Mon, Weds, and Fri: 11:30-12:20 in Thorv 125Laboratory Hours: Mon, Tues: 1.30-5.30 in SG 215

Method of Evaluation:Final Exam (3 hour): 50%Mid-term Exam: 15%Assignments (including oral presentation): 10%Laboratory: 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.

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Symmetry element Symmetry operation Symbol

Identity E

n-Fold symmetry axis Rotation by 2π/n Cn

Mirror plane Reflection σ

Center of inversion Inversion i

n-Fold symmetry axisof improper rotation

Rotation by 2π/n

followed byreflectionperpendicular to rotation axis

Sn

Note that E is a special case of Cn, and that i and σ are special cases of Sn:E = C1, S1 = σ, S2 = i. 4

SO: Proper rotation by 2π/n or 360°/n SE: Axis of rotation Cn

- 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

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SO: Reflection SE: Mirror planes σ- reflection of all atoms through a plane passing through the molecule.

*

σv vertical plane- plane that contains the principal axis

σh horizontal plane- plane perpendicular to the principal axis

σd dihedral plane- plane that contains the principal axisand bisect the angle formed between adjacentC2 axes.

C6

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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.

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SO: Improper rotation by 2π/n SE: n-Fold symmetry axis Sn

- the combination of a rotation by 360°/n followed by reflection across a plane perpendicular to the rotation axis.

S4

S1 = σ S2 = i

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.

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Point group symmetry element shape examples

C1 E SiBrClFI

C2 E, C2 H2O2

Cs E, σ NHF2

C2v E, C2, σv, σv H2O, SO2Cl2

C3v E, C3, 3 σv NH3, PCl3, POCl3C∞v E, C∞, ∞σv CO, HCl, OCS

D2h E, C2(x, y, z), σ(xy,yz,zx), i N2O4, B2H6

D3h E, C3, 3 C2, 3 σv, σh, S3 BF3, PCl5

D4h E, C4, 2 C2’, 2 C2’’, i, S4, σh, 2 σv, 2 σd

D∞h E, C∞, ∞C2, ∞σv, i, S ∞

Td E, 4 C3, 3 C2, 6 σd, 3 S4

Oh E, 3 C4, 4 C3, 6 C2, i, 4 S6, 3 S4, 3 σh, 6 σd

XeF4, trans-MA4B2

H2, CO2, C2H2

CH4, SiCl4

SF6 18

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The Point Groups of Molecules

Determination of the point group of a molecule: method 11. Locate the principal axis of symmetry , this determines the index n.2. Locate secondary axes C2, C3, C4, or C5 at an angle to the principal axis.

This determines the primary symbol C, D, T, O or I.none →C, nC2 →Dn, 4 C3 and C2 →T, 3 C4 and C2, C3 →O, 6 C5 and C2, C3 →I.

3. Identify the mirror planes and their positions with respect to the principal axis.This determines the index h, v or d.

or method 2 : Use the decision tree.

groups of low symmetry: C1, Cs, Ci

groups of n-folded rotation axis: Cn, Cnv, Cnh

dihedral groups: Dn, Dnh, Dnd

groups with very high symmetry: Td, Oh, Ih

Point group Number of SOs

Cn n

Cnv, Cnh, Dn 2 x n

Dnd,Dnh 2 x 2 x n

Td 24

Oh 48

Ih 120

Decision tree for identifying a molecular point group

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Applications of Symmetry

Polar moleculesA 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.

Symmetry forbidden:(with respect to possible dipole moments)CiSnCnh (Cn + σh)Dn (Cn + n C2)Dnh (Cn +n C2 + σh)Dnd (Cn + n C2)Td (4 C3 + 3 C2)Oh (i, C4 + 4 C2, and σh)Ih (i, C5 + 5 C2, and σh)

Symmetry allowed:C1CsCnCnv

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Chiral moleculesA 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).

Applications of symmetry

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 ifit possesses an improper rotation axis Sn

(recall that S1 = σ and S2 = i)

Chiral Achiral (identifying symmetry element)

C1 (asymmetric)Cn (dissymmetric)Dn (dissymmetric)

Cs (plane of symmetry) Ci (center of symmetry)Dnh (plane of symmetry)Dnd (plane of symmetry)Sn (improper axis)Td (plane of symmetry)Oh (center and plane of symmetry)Ih (center and plane of symmetry)Cnv (plane of symmetry)Cnh (center and plane of symmetry)

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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

stretching

bending

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IR Spectroscopy

The four normal modes for CO2.

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.

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Coordination Chemistry

Chapter 19 (Housecroft)

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Coordination Chemistry

Coordination Compound MLn

Central metal atom M surrounded by a set of ligands L. n is the number of ligandsand 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 The atom in the ligand which forms the bond to the centre atom.

Acceptor atom Metal atom or ion, which is the Lewis acid. .

Examples:[Co(NH3)6]Cl3 Trichlorohexaamminecobalt(III)Ni(CO)4 Tetracarbonylnickel[PtCl4]2- Tetrachloroplatinate(II)

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Organometallic ChemistryOne class of coordination compounds which involves metal carbon bonds.

Bioinorganic ChemistryCoordination compounds which are present in living organism.

Coordination Chemistry

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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.

d-orbitals

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Coordination Chemistry

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The problem:4 coordination compounds of cobalt (III) chloride with ammonia has beendiscovered and named according to their colours:

CoCl3·6NH3 yellow Luteo complexCoCl3·5NH3 purple Purpureo complexCoCl3·4NH3 green praseo complexCoCl3·4NH3 violet violeo complex

Addition of Ag+:CoCl3·6NH3 + excess Ag+ → 3AgCl (s)CoCl3·5NH3 + excess Ag+ → 2AgCl (s)CoCl3·4NH3 + excess Ag+ → 1AgCl (s)

Werner’s conclusion:Compound Co(NH3)5Cl3 is derived from compound Co(NH3)6Cl3 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(NH3)6]Cl3, [Co(NH3)5Cl]Cl2, [Co(NH3)4Cl2]Cl, Difference in the conductivity.

Coordination Chemistry

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Werner assigned the correct geometric structure to many coordination compoundslong 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(NH3)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.

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Coordination Chemistry

Geometrical isomerism in six-coordination

[MX4Y2]

[MX3Y3]

mer (for meridional) fac for facial

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Coordination Chemistry

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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 canaccept more electrons from Lewis bases.e.g. [Mo(CN)8]4- octacyanomolybdate(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. [PtCl4]2- tetrachloroplatinate(IV)

Central metal atom could have coordination numbers from 2 to 12.

Coordination Chemistry

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Ligands

monodentate ligands - one point of attachment to the metal atom

Br-, Cl-, OH- H2O, NH3, CO,

polydentate ligands - more than one point of attachment

(CH3COCHCOCH3)- (acac), H2NCH2CH2NH2 (en) , C2O42- (ox)

ambidentate ligands - different potential donor atoms

NO2-, 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.

Coordination Chemistry

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Ligands

The ligand ethylendiamintetraacetato can attach the metal at six points and can form five five-memberedrings. complexometric titration of Ca2+or Zn2+

acetylacetonato

Bite angleL-M-L angle in the chelatea bidentate ligand with a small bite angle can result in distortions from standard structures.

distortion from octahedral totrigonal-prismatic

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Coordination Chemistry

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Chirality and optical isomerism in six-coordination

chiral – not superimposable on its own mirror imageenantiomers – two mirror-image isomers

enantiomers of cis-[CoCl2(en)2] trans-[CoCl2(en)2]

Remember the criterion for chirality is the absence of an axis of improper rotation, which includes the absence of a mirror plane σ = S1 and the absenceof an inversion center i = S2.

Optical isomers

Coordination Chemistry

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right-handscrew

left-handscrew

View along a three-folded rotation axisto determine the handedness of the helix formed by the ligands.

Chelate Effect

The chelate effect or chelation is one of the most important ligand effects in transition metal coordination chemistry. Since most metal-ligandbonds 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:

M L

L

M L

L

+

M L

L

ML

L

ηx

“eta-x” 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.

µx“mu-x” 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.

Rule to fill electrons into p,d,f orbitals containing more than one sublevel of the same energy.

filling p, d, f orbitals: Put electrons into separate orbitals of the subshell with parallel spins before pairing electrons.

The existence of unpaired electrons can be tested for since each acts like a tiny electromagnet.

ParamagneticParamagnetic - attracted to magnetic field. Indicates the presence of unpaired electrons.

DiamagneticDiamagnetic - pushed out of a magnetic field. Indicates that all electrons are paired.

Hund’s Rule

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Molecular Structure And Bonding

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Chapter 4

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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 acidLigand is acting as a Lewis baseUse 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).

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