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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
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
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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+
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
The color of [Ti(H2O)6]3+
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
High-spin and low-spin complex ions of Mn2+
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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