Transcript File

Transition Metal Chemistry and
Coordination Compounds
Lecture 18-20
The Transition Metals
Oxidation States of the 1st Row Transition Metals
(most stable oxidation numbers are shown in red)
Aqueous oxoanions of transition elements
One of the most characteristic
chemical properties of these
elements is the occurrence of
multiple oxidation states.
Mn(II)
Mn(VI)
Mn(VII)
Mn(VII)
Cr(VI)
V(V)
Effects of the metal oxidation state and of ligand identity on color
[V(H2O)6]3+
[V(H2O)6]2+
[Cr(NH3)6]3+
[Cr(NH3)5Cl ]2+
Ionization Energies for the 1st Row Transition Metals
Scandium
Titanium
Vanadium
Chromium
Manganese
Iron
Cobalt
Nickel
Copper
Coordination Compounds
A coordination compound typically consists of a complex ion
and a counter ion.
A complex ion contains a central metal cation bonded to one
or more molecules or ions.
The molecules or ions that surround the
metal in a complex ion are called ligands.
H
H
H H H
••
Cl
••
-
C
••
O
••
••
••
N
••
A ligand has at least one unshared pair
of valence electrons
O
22.3
Coordination Compounds
The atom in a ligand that is bound directly to the metal atom is
the donor atom.
••
N
O
H
H
H H H
The number of donor atoms surrounding the central metal atom
in a complex ion is the coordination number.
Ligands with:
one donor atom
two donor atoms
three or more donor atoms
monodentate
bidentate
H2O, NH3, Clethylenediamine
polydentate
EDTA
Coordination Compounds
bidentate ligand
••
H2N
CH2
CH2
••
NH2
polydentate ligand
(EDTA)
Bidentate and polydentate ligands are called chelating agents
EDTA Complex of Lead
What are the oxidation numbers of the metals in
K[Au(OH)4] and [Cr(NH3)6](NO3)3 ?
OH- has charge of -1
K+ has charge of +1
? Au + 1 + 4x(-1) = 0
Au = +3
NO3- has charge of -1
NH3 has no charge
? Cr + 6x(0) + 3x(-1) = 0
Cr = +3
Naming Coordination Compounds
•
The cation is named before the anion.
•
Within a complex ion, the ligands are named first in
alphabetical order and the metal atom is named last.
•
The names of anionic ligands end with the letter o. Neutral
ligands are usually called by the name of the molecule. The
exceptions are H2O (aqua), CO (carbonyl), and NH3
(ammine).
•
When several ligands of a particular kind are present, the
Greek prefixes di-, tri-, tetra-, penta-, and hexa- are used to
indicate the number. If the ligand contains a Greek prefix,
use the prefixes bis, tris, and tetrakis to indicate the number.
•
The oxidation number of the metal is written in Roman
numerals following the name of the metal.
•
If the complex is an anion, its name ends in –ate.
22.3
What is the systematic name of
[Cr(H2O)4Cl2]Cl ?
tetraaquodichlorochromium(III) chloride
Write the formula of
tris(ethylenediamine)cobalt(II) sulfate
[Co(en)3]SO4
Crystal-Field Theory
• Model explaining bonding for transition
metal complexes
– • Originally developed to explain properties for
crystalline material
– • Basic idea:
– Electrostatic interaction between lone-pair electrons
result in coordination.
Energetics
• CFT - Electrostatic between
metal ion and donor atom
i) Separate metal and ligand high
energy
ii) Coordinated Metal - ligand stabilized
iii) Destabilization due to ligand -d
electron repulsion
iv) Splitting due to octahedral field.
i
ii
iv
iii
d-Orbitals and Ligand
Interaction
(Octahedral Field)
•Ligand
s
approac
h metal
d-orbitals pointing directly at axis are
affected most by electrostatic interaction
d-orbitals not pointing directly at axis are least affected
(stabilized) by electrostatic interaction
Ligand-Metal Interaction
•Crystal Field Theory - Describes
bonding in Metal Complexes
• Basic Assumption in CFT:
• Electrostatic interaction between ligand
and metal
d-orbitals align along the octahedral axis
will be affected the most.
More directly the ligand attacks the
metal orbital, the higher the energy of
the d-orbital.
In an octahedral field the degeneracy of
the five d-orbitals is lifted
Splitting of the d-Orbitals
• Octahedral field Splitting Pattern:
•
The energy gap is
referred to as
(10 Dq) , the
crystal field
splitting energy.
The dz2 and dx2y2 orbitals lie on the same axes as negative charges.
Therefore, there is a large, unfavorable interaction between ligand (-) orbitals.
These orbitals form the degenerate high energy pair of energy levels.
The dxy , dyx and dxz orbitals bisect the negative charges.
Therefore, there is a smaller repulsion between ligand & metal for these orbitals.
These orbitals form the degenerate low energy set of energy levels.
Splitting of d orbitals in an octahedral field
eg
3/5 o
o
2/5 o
t2g
o is the crystal field splitting
E(t2g) = -0.4o x 3 = -1.2o
E(eg) = +0.6o x 2 = +1.2o
The magnitude of the splitting
(ligand effect)
Strong
field
Weak
field
The spectrochemical series
CO, CN- > phen > NO2- > en > NH3 > NCS- > H2O > F- > RCO2- > OH- > Cl- > Br- > I-
The magnitude of the splitting
(metal ion effect)
Strong
field
Weak
field
 increases with increasing formal charge on the metal ion
 increases on going down the periodic table
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
SMALLER 
LONGER 
STRONGER FIELD
LARGER 
SHORTER 
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Electron Configuration in Octahedral
Field
• Electron configuration of metal ion:
•
s-electrons are lost first.
•
Ti3+ is a d1, V3+ is d2 , and Cr3+ is d3
• Hund's rule:
•
First three electrons are in separate
d orbitals with their spins parallel.
• Fourth e- has choice:
•
Higher orbital if  is small; High
spin
•
Lower orbital if  is large: Low
spin.
• Weak field ligands
•
Small  , High spin complex
• Strong field Ligands
•
Large  , Low spin complex
Placing electrons in d orbitals
Strong field
Weak field
Strong field
Weak field
d1
d2
d3
d4
When the 4th electron is assigned it will either go into the higher
energy eg orbital at an energy cost of 0 or be paired at an energy
cost of P, the pairing energy.
d4
Strong field =
Low spin
(2 unpaired)
Weak field =
High spin
(4 unpaired)
P < o
P > o
Pairing Energy, P
The pairing energy, P, is made up of two parts.
1) Coulombic repulsion energy caused by having two electrons in
same orbital. Destabilizing energy contribution of Pc for each
doubly occupied orbital.
2) Exchange stabilizing energy for each pair of electrons having the
same spin and same energy. Stabilizing contribution of Pe for
each pair having same spin and same energy
P = sum of all Pc and Pe interactions
Placing electrons in d orbitals
d5
1 u.e.
5 u.e.
d6
0 u.e.
4 u.e.
d8
2 u.e.
2 u.e.
d7
1 u.e.
3 u.e.
d9
1 u.e.
1 u.e.
d10
0 u.e.
0 u.e.
To sum up
•Electron Configuration for Octahedral
complexes of metal ion having d1 to d10
configuration [M(H2O)6]+n.
•Only the d4 through d7 cases have both high-spin and
low spin configuration.
Electron configurations for
octahedral complexes of
metal ions having from d1
to d10 configurations. Only
the d4 through d7 cases
have both high-spin and
low-spin configurations.
Colour in Coordination Compounds
E = hn
22.5
The absorption maximum for the complex ion
[Co(NH3)6]3+ occurs at 470 nm. What is the color of
the complex and what is the crystal field splitting in
kJ/mol?
Absorbs blue, will appear orange.
hc (6.63 x 10-34 J s) x (3.00 x 108 m s-1)
 = hn =
=
= 4.23 x 10-19 J

470 x 10-9 m
 (kJ/mol) = 4.23 x 10-19 J/atom x 6.022 x 1023 atoms/mol
= 255 kJ/mol
22.5
Color Absorption of Co3+
Complexes
of
Color of Light
Color of Complex
• The Wavelength
Colors
of
Some
Complexes
light absorbed
Absorbed
3+ Ion
3+ the 700
Co(nm)
[CoF6]of
Red
Green
Complex Ion
[Co(C2O4)3] 3+
600, 420
Yellow, violet
Dark green
[Co(H2O)6] 3+
600, 400
Yellow, violet
Blue-green
[Co(NH3)6] 3+
475, 340
Blue, violet
Yellow-orange
[Co(en)3] 3+
470, 340
Blue, ultraviolet
Yellow-orange
[Co(CN)6] 3+
310
Ultraviolet
Pale Yellow
The complex with fluoride ion, [CoF6]3+ , is high spin and has one absorption band.
The other complexes are low spin and have two absorption bands. In all but one
case, one of these absorptionsis in the visible region of the spectrum. The
wavelengths refer to the center of that absorption band.
Colors & How We
Perceive it
650
580
800
560
400
Artist color wheel
showing the colors which
are complementary to one
another and the wavelength
range of each color.
430
490
Complex Influence on Color
•Compounds of Transition metal
complexes solution.
650
580
800
400
430
[Fe(H2O)6
]3+
[Ni(H2O)6]2+
[Co(H2O)6]2+
[Zn(H2O)6]2+
[Cu(H2O)6]2+
560
490