Chemistry Analysis What is this? Modeling How do I explain it? Synthesis How do I make it? Three central goals.

Chemistry

AnalysisWhat is this?

ModelingHow do I explain it?

SynthesisHow do I make it?

Three central goals

A Chemist’s View- How A Chemist’s View- How we thinkwe think

Macroscopic

Microscopic orParticulate

Symbolic

NaCl

Three different

perspectives

The Period 4 transition metals

Colors of representative compounds of the Period 4 transition metals

titanium oxide

sodium chromate

potassium ferricyanide

nickel(II) nitrate hexahydrate

zinc sulfate heptahydrate

scandium oxide

vanadyl sulfate dihydrate

manganese(II) chloride

tetrahydrate cobalt(II) chloride

hexahydrate

copper(II) sulfate

pentahydrate

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Aqueous oxoanions of transition elements

Mn(II) Mn(VI) Mn(VII)

V(V)Cr(VI)

Mn(VII)

One of the most characteristic chemical properties of these elements is the occurrence of multiple oxidation states.

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Effects of the metal oxidation state and of ligand identity on color

[V(H2O)6]2+ [V(H2O)6]3+

[Cr(NH3)6]3+ [Cr(NH3)5Cl ]2+

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Linkage isomers

An artist’s wheel

Splitting of d-orbital energies by an octahedral field of ligands

is the splitting energy

The effect of ligand on splitting energy

The spectrochemical series

•For a given ligand, the color depends on the oxidation state of the metal ion.

•For a given metal ion, the color depends on the ligand.

I- < Cl- < F- < OH- < H2O < SCN- < NH3 < en < NO2- < CN- < CO

WEAKER FIELD STRONGER FIELD

LARGER SMALLER

LONGER SHORTER

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The color of [Ti(H2O)6]3+

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High-spin and low-spin complex ions of Mn2+

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Orbital occupancy for high- and low-spin complexes of d4 through d7 metal ions

high spin: weak-field

ligand

low spin: strong-field

ligand

high spin: weak-field

ligand

low spin: strong-field

ligand

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What is electronic spectroscopy?

Absorption

Absorption of radiation leading to electronic transitions within a molecule or complex

UV = higher energy transitions - between ligand orbitals

visible = lower energy transitions - between d-orbitals of transition metals

- between metal and ligand orbitals

UV

400

nm (wavelength)

200 700

visible

Absorption

~14 000 50 00025 000

UVvisible

cm-1 (frequency)

[Ru(bpy)3]2+ [Ni(H2O)6]2+

10104

Absorption maxima in a visible spectrum have three important characteristics

1. number (how many there are)

This depends on the electron configuration of the metal centre

2. position (what wavelength/energy)

This depends on the ligand field splitting parameter, oct or tet and on the degree

of inter-electron repulsion

3. intensity

This depends on the "allowedness" of the transitions which is described by two

selection rules

eg

t2g

o

h

d-d transition

[Ti(OH2)6]3+ max = 510 nm o is 243 kJ mol-1

20 300 cm-1

The energy of the absorption by [Ti(OH2)6]3+ is the ligand-field splitting, o

An electron changes orbital; the ion changes energy state

complex in electronic

Ground State (GS)

complex in electronic

excited state (ES)

GS

ES

GS

ES

eg

t2g

A

/ cm-1-30 00020 00010 000

[Ti(H2O)6]3+, d1

2T2g

2Eg

2B1g

2A1g

The Jahn-Teller Distortion: Any non-linear molecule in a degenerate electronic state

will undergo distortion to lower it's symmetry and lift the degeneracy

d3 4A2g

d5 (high spin) 6A1g

d6 (low spin) 1A1g

d8 3A2g

Degenerate electronic ground state: T or E

Non-degenerate ground state: A

Limitations of ligand field theory

LFT assumes there is no inter-electron repulsion

[Ni(OH2)6]2+ = d8 ion

2+

Ni

A

3 absorption bands

eg

t2g

Repulsion between electrons in d-orbitals has an effect on the energy of the whole ion

15 00025 000cm-1

Electron-electron repulsiond2 ion

eg

t2g

xy xz yz

z2 x2-y2eg

t2g

xy xz yz

z2 x2-y2

xz + z2 xy + z2

lobes overlap, large electron repulsion lobes far apart, small electron repulsion

x

z

x

z

yy

These two electron configurations do not have the same energy

- some covalency in M-L bonds – M and L share electrons

-effective size of metal orbitals increases

-electron-electron repulsion decreases

Nephelauxetic series of ligands

F- < H2O < NH3 < en < [oxalate]2- < [NCS]- < Cl- < Br- < I-

Nephelauxetic series of metal ions

Mn(II) < Ni(II) Co(II) < Mo(II) > Re (IV) < Fe(III) < Ir(III) < Co(III) < Mn(IV)

cloud expandingThe Nephelauxetic Effect

Selection Rules

Transition complexes

Spin forbidden 10-3 – 1 Many d5 Oh cxsLaporte forbidden [Mn(OH2)6]2+

Spin allowedLaporte forbidden 1 – 10 Many Oh cxs

[Ni(OH2)6]2+

10 – 100 Some square planar cxs [PdCl4]2-

100 – 1000 6-coordinate complexes of low symmetry, many square planar cxs particularly with organic ligands

Spin allowed 102 – 103 Some MLCT bands in cxs with unsaturated ligandsLaporte allowed

102 – 104 Acentric complexes with ligands such as acac, or with P donor atoms

103 – 106 Many CT bands, transitions in organic species