Download PPT File

University Chemistry
Chapter 15: The Chemistry of
Transition Metals
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
What are the transition metals ?
Element with typical electron configurations ns2 (n-1)dx
incompletely filled d orbitals
Properties
“metallic” due to loosely bound ns electrons.
Various colors
Various reduction/oxidation potentials
Possess catalytic activities
3
4
17.1 The d-block Metals: Energies, Charge States and Ioinc Radii
Charge states (oxidation states)
Early transition metals
Late transition metals
High oxidation states
do not mean
the ionic charges
Early d elements:
of the metals.
Formation of oxyions
Just oxidation
by polar-covalent
numbers!
bonding with O, Cl, F
Late d elements:
resistant to further oxidation
beyond M2+ or M3+
*
Metal ions (mostly coordination complex ions)
**
Ionization Energies for the 1st Row Transition Metals
6
17.1 The d-block Metals: Energies, Charge States and Ioinc Radii
Ionic radii
Similar to neutral ones.
Not a so smooth change as for the p elements.
17.2 chemistry of the early transition metals: oxyions
Oxyions : metals combined with oxygen to form a polyatomic molecular ion.
examples
Cr2O72-
MnO4-
Oxidizing agents
permanganate
dichromate
CrO42- + 2H+
Cr2O72-
+ H2O
H2SO4
CrO3
Sc2O3
TiO2
H2O
H2CrO4
V2O5
Oxidation states: higher oxidation state– more covalent bond character
lower oxidation state – more ionic bond character
Mn(OH)2, Mn(OH)3, H2MnO3, H2MnO4, HMnO4
basic
acidic
17.2 chemistry of the early transition metals: oxyions
Spectroscopy and structure of oxyanions
Comparison of isoelectronic species
VO4
3-
CrO4
2-
MnO4
Follows octet rule.
Tetrahedral structure
according to VSEPR.
-
Absorption spectra
*
17.2 chemistry of the early transition metals: oxyions
VO43-
CrO42-
MnO4-
Molecular orbitals
4p and 4s AO are spatially
more extended than 3d,
to interact with oxygen MO’s.
3d AO’s remain as
nonbonding.
follow octet rule,
Tetrahedral structure
17.2 chemistry of the early transition metals: oxyions
VO43-
CrO42-
MnO4-
follow octet rule,
Tetrahedral structure
Molecular orbitals
Charge-transfer transitions : electron jumps from O to M like orbitals.
This can absorb light very strongly.
17.3 chemistry of the late transition metals: coordination complexes
Stoichiometry, Isomerism, and Geometry of Complexes
Chemical formula(19thC.)
Color
Chemical formula (Werner)
Isomers
CoCl3.6NH3
orange-yellow
[Co(NH3)6]3+Cl-3
1
CoCl3.5NH3
purple
[Co(NH3)5Cl]2+Cl-2
1
CoCl3.4NH3
green
[Co(NH3)4Cl2]+Cl-
2
CoCl3.3NH3
green
[Co(NH3)3Cl3]
1
Octahedral
structure
cis
trans
These two are geometrical isomers
17.3 chemistry of the late transition metals: coordination complexes
Oxidation numbers of 2 and 3 (1 is possible for Cu).
Formation of common oxides: Fe3O4, Fe2O3, CoO, Co3O4, NiO, Cu2O, CuO, ZnO
Oxides are easily soluble in acid to form colored solution. d10(Cu, Zn) are colorless.
Color of aqueous solution of ions
Fe2+(aq)
Fe3+(aq)
Co2+(aq)
Co3+(aq)
CuSO4 : greenish white v.s.
Ni2+(aq)
Cu2+(aq)
CuSO4.4H2O : blue
These colors are due to the formation of coordination complexes.
Coordination complex : Cu(H2O)42+
n+
M
n+
+
mL
Ligand
coordination
Making coordination complex
charge of a complex = sum of charges of metals and ligands
charge of a complex + charges of counter ions = 0
coordination number = numbers of donor atoms
M Lm
coordination complex
m : coordination number
Coordination Compounds
A coordination compound typically consists of a complex ion
and a counter ion.
Primary valence corresponds to the oxidation number and
secondary valence to the coordination number of the element.
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
14
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
15
bidentate ligand
••
H2N
CH2
CH2
••
NH2
polydentate ligand
(EDTA)
Bidentate and polydentate ligands are called chelating agents
16
17
EDTA Complex of Lead
Net charge of a complex ion is the sum of the charges on the
central metal atom and its surrounding ligands.
Pb2+
EDTA4-
Complex: 218
19
17.3 chemistry of the late transition metals: coordination complexes
Nomenclature
1. Cation
Anion
b
2. In the complex: names of ligands come first and then name of metal.
Among ligands: alphabetical order.
3. Names of ligands: anion – change the last letter to o.
neutral – same as the original ones.
4. Counting number of ligands: di, tri, tetra, penta, hexa, hepta…..
if the ligand contains these names in it, use: bis, tris, tetrakis, pentakis……
5. If the compex is an anion: at the end of the name put ate.
6. Oxidation number of metal: in parenthesis with Roman letter - (IV).
Examples
[Co(NH3)5Cl]Cl2
Pentaamminechlorocobalt(III) chloride
K4[Fe(CN)6]
Potassium hexacyanoferrate(II)
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 (aquo), 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.
21
22
23
24
25
Structure of Coordination Compounds
Coordination number
2
Structure
Linear
4
Tetrahedral or Square planar
6
Octahedral
26
17.3 chemistry of the late transition metals: coordination complexes
Structure of coordination complexes
[Ag(NH3)2]+
coordination
number
2
structure
Linear
Atomic orbital
of metal
d10
4
Tetrahedral
d10
[Pt(NH3)4]2+
4
Square Planar
d8
[Co(NH3)6]3+
6
Octahedral
d6
[Zn(NH3)4]2+
*(b) and (c) are geometrical isomers.
Stereoisomers are compounds that are made up of the same
types and numbers of atoms bonded together in the same
sequence but with different spatial arrangements.
Geometric isomers are stereoisomers that cannot be
interconverted without breaking a chemical bond.
cis-[Pt(NH3)2Cl2]
trans-[Pt(NH3)2Cl2]
28
cis-[Co(NH3)4Cl2]
trans-[Co(NH3)4Cl2]
Rotate 90o
Rotate 90o
trans
cis
same compounds
same compounds
29
Optical isomers are nonsuperimposable mirror images.
Rotate 180o
cis-[Co(en)2Cl2]
trans-[Co(en)2Cl2]
optical isomers
not optical isomers
chiral
achiral
30
17.3 chemistry of the late transition metals: coordination complexes
Multidentate ligands (or chelating ligands)
bidentate
ligands
tetradentate
hexadentate
These are non-superimposable mirror images
to each other. So they are optical isomers.
Coordination Compounds in Living Systems
The porphine molecule plays an important
role in some biological compounds.
hemoglobin
Cytochrome c
chlorophyll
32
17.3 chemistry of the late transition metals: coordination complexes
Magnetism: paramagetic v.s. diamagnetic
(paramagnetic if there are unpaired electrons)
Zn2+
d10
always diamagnetic – no unpaired electrons
[Co(NH3)6]3+
d6
diamagnetic – no unpaired electrons
low spin complex
[CoF6]3-
d6
paramagnetic – 4 unpaired electrons
high spin complex
Lability: ability to undergo a reaction (should meet both the
kinetic and thermodynamic requirements).
Some ligands bind to metals more tightly than others, replacing weakly bound ligands.
Replacement of a strong ligand by a weak one is labile.
CO, CN-, H2NCH2CH2NH2 > H2O
[Ni(H2O)6]2+
+
6NH3
[Ni(NH3)6]2+
+ 6H2O
[Ni(en)3]2+
+ 6H2O
Kf = 4 x 108
[Ni(H2O)6]2+
+
3en
Kf = 2 x 1018
Kf is called the formation constant of a complex (see Table 17.3).
17.4 The spectrochemical series and bonding in complex
Color changes of complexes
NiCl2
HCl(12M)
NiCl64-
H2O
Ni(H2O)6
2+
NH3
Ni(NH3)62+
en
Ni(en)32+
more stable
Spectrochemical Series:
Arrangement of ligands in order of increasing stability of complex.
Magnetic properties also follow this series.
strong field ligands
weak field ligands
Example: Fe(H2O)62+
paramagetic
a high-spin complex
Example: Fe(CN)64diamagnetic
a low-spin complex
Strong-field ligands form a high-spin complex that are paramagnetic,
whereas weak-field ligands form a paramagnetic low-spin complex.
17.4 The spectrochemical series and bonding in complex
Are there ways to explain and eventually predict colors, spectrochemical series
and magnetism?
Crystal field theory : ionic description of the metal-ligand bonds
Only considering the electrostatic interaction between ligand and metal atom:
charge-charge, charge-dipole.
Then, consider changes in the energy levels of metal d orbitals according to
the interaction during coordination.
Let’s Begin with octahedral geometry
Bonding in Coordination Compounds
Crystal field theory explains the bonding in complex ions
purely in terms of electrostatic forces.
• the attraction between the positive metal ion and the
negatively charged ligand or the partially negatively
charged end of a polar ligand
• electrostatic repulsion between the lone pairs on the
ligands and the electrons in the d orbitals of the metals
All d orbitals equal in energy in the absence of ligands! 37
Splitting in Octahedral Complexes
Isolated transition
metal atom
Bonded transition
metal atom
The lobes of the d x 2  y 2 and d 2are
z
pointed directly at the ligands,
increasing their energy.
Crystal field splitting ( D ) is the energy difference between
38
two sets of d orbitals in a metal atom when ligands are present
Crystal field theory for the octahedral structure
Do : crystal
field splitting energy
Overall stabilization through splitting:
crystal field stabilization energy (CFSE)
Color of Coordination Compounds
Absorbs all wavelengths: Black
Transmits all wavelengths: Colorless
(white)
If one color is absorbed, the complementary color is seen.
A solution of CuSO4 absorbs orange wavelengths
so the solution appears blue.
Energy of absorbed photon = D
40
Different values of D, result in different colors exhibited by
complex ions.
Aquo complexes of first row transition metal ions.
Ti3+
Cr3+
Mn2+
Fe3+
Complexes will be colorless if no
light is absorbed or if the absorbed
wavelength is not in the visible
region.
Co2+
Ni2+
Cu2+
41
Spectroscopic Determination of D
l=498 nm
42
Spectrochemical Series
A list of ligands arranged in increasing order of their abilities to
split the d-orbital energy levels.
I- < Br- < Cl- < OH- < F- < H2O < NH3 < en < CN- < CO
increasing D
Weak-field ligands
Small D
Strong-field ligands
Large D
43
Magnetic Properties
weak-field ligand
strong-field ligand
The arrangement of the electrons is determined by the stability
gained by having maximum parallel spins versus the investment
in energy required to promote electrons to higher d orbitals.
Actual number of unpaired electrons can be determined
by electron spin spectroscopy ( ESR).
44
Orbital diagrams for the highspin and low-spin octahedral
complexes corresponding
to the electron configurations d4,
d5, d6, and d7.
No such distinctions can be
made for d1, d2, d3, d8, d9, and
d10.
45
Summary of Octahedral Complexes
magnetism
d1 ~ d5 : always paramagnetic
d7 ~ d9 : always paramagneticd6 :
d10 :
always diamagnetic
depending on the ligands
Tetrahedral Complexes ???
Reversal of octahedral !!!
M
Tetrahedral complexes
Reversal of octahedral !
Splitting in Tetrahedral Complexes
The dxy, dyz, and dxz orbitals are
more closely directed at the
ligands
49
Square planar???
Removal of axial ligands from octahedral
Removal of axial ligands from octahedral
Splitting in Square Planar Complexes
The d x 2  y 2 orbital possesses the
highest energy and the dxy orbital
the next highest. However, the
relative placement of the d z and
the dxz and dyz orbitals must be
calculated.
52
2
Weak point of crystal field theory
1. Coordination is not fully ionic.
2. Spectrochemical series is all empirical.
3. Does not consider nature of the ligands.
Ligand field theory
Ligand Field Theory
Molecular orbital approach to the electronic structure of
coordination compounds.
Based on the idea that atomic orbitals that are close in
energy will mix more effectively in molecular orbitals than those that
are far apart.
d x 2  y 2and d z 2 orbitals on the metal center will mix with the ligand
lone-pair orbitals to form two bonding and two antibonding
molecular orbitals.
The remaining d orbitals—dxy, dyz and dxz—are oriented in
between the ligands and will remain nonbonding orbitals
54
Arrangement of the highest
occupied molecular orbitals (3dxy,
3dyz, 3dxz, and the two s*d orbitals)
is identical to that predicted by
crystal field theory
Provides an understanding of the
dependence of the crystal field
splitting on the ligand type.
55
Now, we can explain Spectrochemical Series
I- < Br- < Cl- < F-, OH- < H2O < NCS- < NH3 < en < CO, CNWeak field Ligands
small D o
Interaction between dxy of metal and py of halide:
charge repulsion
Increases energy level of t2g
Strong field Ligands
large D o
pback-bonding of ligand can overlap
pwith dxy orbital
Lowers the energy level of t2g
p back-bonding
Makes smaller D o for I- and less smaller one for F-
Makes larger Do for CO, CN-
Ligand exchange (or substitution) reactions
kinetic lability - tendency to react
instantaneous
*CN:
14C
labeled
labile complex—undergo rapid ligand exchange reactions.
A thermodynamically stable species (that is, one that has a
large formation constant) is not necessarily unreactive.
several days
inert complex—a complex ion that undergoes very slow
exchange reactions
A thermodynamically unstable species is not necessarily
chemically reactive.
57
Applications of Coordination Compounds
Metallurgy
extract and purify metals,
Therapeutic Chelating Agents
EDTA is used in the treatment of lead poisoning. Certain
platinum-containing compounds can effectively inhibit the
growth of cancerous cells.
58
Chemical Analysis
bis(dimethylglyoximato)nickel(II)
dimethylglyoxime
Characteristic colors are used in qualitative analysis to identify
nickel and palladium.
Detergents
Tripolyphosphate ion is an effective chelating agent that
forms stable, soluble complexes with Ca2+ ions.
59
Cisplatin – The Anticancer Drug
Cisplatin works by chelating DNA (deoxyribonucleic acid), the
molecule that contains the genetic code.
Consequently, the double-stranded structure assumes a bent
configuration at the binding site which is thought to inhibit 60
replication.