Download Transition metals and coordination chemistry

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Transition metals and coordination chemistry
Physical properties: (1) metallic luster (2) high electrical and thermal
conductivity (3) wide range of melting points (W: 3400oC, Hg: <25oC)
(4) hardness (5) wide range of reactivity towards O2 (forming oxides)
(Fe reacts easily with O2 while coinage metals do not react with O2)
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24
25
26
27
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Ti
V
Cr
Mn
Fe
Co
Ni
3d14s2
3d24s2
3d34s2
3d54s1
3d54s2
3d64s2
3d74s2
3d84s2
45
45
Y
Zr
Nb
Mo
Tc
Ru
Rh
Pd
[Kr]
4d15s2
4d25s2
4d45s1
4d55s1
4d55s2
4d75s1
4d85s1
4d10
La
Hf
Ta
W
Re
Os
Ir
Pt
[Xe]
5d16s2
4f145d26s2
4f145d36s2
4f145d46s2
4f145d56s2
4f145d66s2
4f145d76s2
4f145d96s1
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Show the periodic table (oxidation states and electron configurations)
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Sc
[Ar]
57
40
41
72
42
73
74
To write the electronic structure for Co2+:
Co
Co2+
[Ar] 3d74s2
[Ar]
3d7
43
44
75
76
77
78
To write the electronic structure for V3+:
V
[Ar] 3d34s2
V3+
[Ar] 3d2
Titanium (0.6% by mass of Earth's crust)
Titanium metal - characterized by low density, high strength,
chemical inertness- used as a structural element in high-demand
uses such as jet engines, high-performance bicycles, etc.
Chromium (Cr) - (greek chroma, color)
Cr(0) (metal) is used for making steel. Cr(VI) and Cr(III) are
used for chrome plating, dyes and pigments, leather tanning,
and wood preserving.
Chromium enters the air, water, and soil mostly as Cr(III) and
Cr(VI). Cr(III) strongly attaches to soil, but a small amount can
dissolve in water and move deeper to underground water
reservoirs - pollutant.
The colors of many
gemstones come from
Cr(III) impurities
Ruby: Cr3+ in Al2 O3
Large crystals of copper sulfate
The Copper(II) sulfate salt exists as a series of compounds that differ in their
degree of hydration. The anhydrous form is a pale green or gray-white powder,
whereas the pentahydrate, the most commonly encountered salt, is bright
blue.
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Coordination Chemistry
The Nobel Prize in Chemistry 1913
Alfred Werner (University of Zurich, Switzerland)
"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“
Prior to Werner's work, it was not known how the atoms in a
molecule of the chemical formula [Pt(NH3)2Cl2] were connected.
The theories at the time predicted such molecules to be connected
as linear chains: [Pt-NH3-NH3-Cl]Cl or Cl-NH3-Pt-NH3-Cl
Compound
Color
Historic name
CoCl3 • 6 NH3
Yellow
Luteo-complex
CoCl3 • 5 NH3
purple
Purpureo-complex
CoCl3 • 4 NH3
Green
Praseo-complex
CoCl3 • 4 NH3
Violet
Violeo-complex
Werner studied the conductivities of solutions. Other factors being
equal, the conductivity of an aqueous solution of an ionic salt is
proportional to the number of ions produced when the salt is dissolved.
Werner studied the following series:
Pt(NH3)6Cl4
has relatively high conductivity (more ions)
Pt(NH3)5Cl4
has decreasing conductivity
Pt(NH3)4Cl4
has decreasing conductivity
Pt(NH3)3Cl4
has decreasing conductivity
Pt(NH3)2Cl4
has zero conductivity
KPt(NH3)Cl5
has increasing conductivity
K2PtCl6
has increasing conductivity
These experiments led Werner to postulate the presence of 2 different
types of bonding in inorganic coordination compounds:
1. Primary valences, fixed in number by charge balance; these
were ionic (i.e. to ions only) and non-directional.
2. Secondary valences, determined by the nature of the central
metal atom (usually 4 or 6); non-ionic, directional.
Based on the postulations, Werner formulated the compounds
[Pt(NH3)6]Cl4
[Pt(NH3)5Cl]Cl3
[Pt(NH3)4Cl2]Cl2
[Pt(NH3)3Cl3]Cl
[Pt(NH3)2Cl4]
K[Pt(NH3)Cl5]
K2[PtCl6]
has relatively high conductivity (more ions)
has decreasing conductivity
has decreasing conductivity
has decreasing conductivity
has zero conductivity
has increasing conductivity
has increasing conductivity
(4-n)+
[Pt(NH3)6-nCln]
1-
(Cl )4-n
Coordination chemistry
Transition metals can form a variety of coordination compounds
Complex: A central metal ion (atom) bonded via coordinate bonds
to one or more molecules or ions called ligands
Complex ion – complex with net charge
coordination number – number of coordinate bonds between metal
ion and the ligand or ligands
Coordination compound: (1) neutral substance made up of complex
ion and another ion of opposite charge.
(2) neutral complex
Ligand: contains at least one nonbonding pair of electrons
Some important aspects of transition metal ions
(1) The valence electrons are in d orbitals
(2) The d orbitals do not have large radial extension. They are therefore
mostly nonbonding in complexes of transition metal ions
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Valence bond description
a ligand-to-metal dative bond
metal center
M
metal empty
hybridized orbital
L ligand
ligand orbital
accomodating a lone pair of electrons
Ligands
Bidentate ligands (chelate ligands)
Types of Ligands
• Depending on the number of electron lone pairs available,
ligands will bond to a metal through 1 (monodentate), 2
(bidentate), 3 (tridentate), ... atoms
• Any polydentate ligand is also called a chelating ligand
• Cl-, NH3, H2O are all monodentate ligands
• Ethylenediamine (en) is a common bidentate ligand
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Polydentate ligands
ethylenediaminetetraacetic acid
H4EDTA
In 1999, the annual consumption of EDTA was equivalent to about 35,000
tons in Europe and 50,000 tons in the US.
The most important uses are:
Industrial cleaning: complexation of Ca2+ and Mg2+ ions, binding of heavy
metals.
Detergents: complexation of Ca2+ and Mg2+ (reduction of water hardness).
Photography: use of Fe(III)EDTA as oxidizing agent.
Pulp and paper industry: complexation of heavy metals during chlorine-free
bleaching, stabilization of hydrogen peroxide.
Textile industry: complexation of heavy metals, bleach stabilizer.
EDTA is used in chelation therapy for acute hypercalcemia (急性的高血鈣症),
mercury poisoning and lead poisoning
Classify the following ligands as monodentate, bidentate, or tridentate.
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The Concept of Coordination Numbers
The concept of coordination numbers was an important result of Werner's
research. It clearly established the two most important aspects that are
required to discuss chemical structure and change:
oxidation state and coordination number.
The concept of oxidation state implies that we have knowledge who
loses electrons and who gains electrons in a compound. This cannot be
done unambiguously unless generally accepted rules are applied.
Coordination number -- 1
Complexes with coordination
number 1 were erroneously
claimed for copper and silver,
but a genuine metal complex
with CN= 1 (M = Indium) was
recently reported by P. Power
(S. T. Haubrich, P. Power,
JACS 1998, 120, 2202-2203)
i
Pr
i
Pr
i
Pr
In
i
Pr
i
Pr
i
Pr
The concept of coordination number is a geometric concept and simply
represents the number of next neighbors. It is important to realize that the
concept of coordination numbers does not imply any specific type of
bonding. Dative bonds, hydrogen bridge bonding, single, double or triple
bonds indeed any type of bond (including those found in solids) is
permissible. It is this generality of the concept that makes the concept so
valuable.
Stable compounds
with coordination
number 2 are
restricted to the
copper group (Cu,
Ag, Au) and to
mercury.
Typical
coordination
numbers
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Isomerism
Isomers
(same formula, different properties)
Structural isomers
(different bonds)
Coordinationsphere isomers
Linkage isomers
Stereoisomers
(same bonds,
different arrangements)
Geometric
isomers
Cis and trans
isomers
Structural isomers
Optical isomers
Mirror images
Linkage isomers
Linkage isomers are two or more coordination compounds in which the
donor atom of at least one of the ligands is different (i.e., the connectivity
between atoms is different)
Two types of structural isomers are linkage isomerism and
coordination-sphere isomers.
Coordination sphere isomers: ratio of ligand:metal same, but
ligands are attached to metal ions in different numbers
–
–
–
–
[Cr(H2O)6]Cl3, violet
[CrCl(H2O)5]Cl2.H2O, blue-green
[CrCl2(H2O)4]Cl.2H2O, dark green
[CrCl3(H2O)3].3H2O, yellow green
3+
OH2
H2 O
Cr
[CoIII(NH3)5Br]SO4
[CoIII(NH3)5SO4] Br
2+
Cl
OH2
H2O
OH2
H2 O
violet
-
3Cl
H2 O
Cr
H2 O
OH2
OH2
H2 O
green
2Cl-
Similarly, SCNcan coordinate
through S or N.
+
Cl
H2 O
Cr
H2 O
OH2
OH2
Cl-
trans-[RuCl(py)4(NO)]+
Cl
Phys. Chem. Chem. Phys.,
2007, 9, 3717 - 3724
green
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Stereoisomers
Geometric isomers
Stereoisomers are isomeric molecules whose atomic connectivity is the
same but whose atomic arrangement in space is different.
Geometric isomers
Geometric isomers are stereoisomers where the two forms are not mirror
images of each other.
Square planar complexes
Pt(NH3)2Cl2 exists
as cis and trans
isomers
Octahedral complexes
fac (facial)
mer (meridional)
cis
trans
Tetrahedral
complexes have
all positions
equivalent and
adjacent to all
other positions,
so there are no
geometric
isomers
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Optical isomers
Non-identical mirror images with the same geometric arrangement of ligands.
A set of two (mirror image) isomers are called enantiomers.
Enantiomers differ only in optical properties – rotate plane-polarized light in
opposite directions.
Optical isomers are mirror images of one another that cannot be superimposed.
Neither tetrahedral M(A-A)2 nor square planar M(A-A)2 has isomers
Tetrahedral M(A-B)2 has optical isomers but not geometric isomers;
square planar M(A-B)2 has geometric isomers but not optical isomers.
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Crystal field theory (Ligand field theory) – Color and Magnetism
Most transition metal complexes are colorful
Color of a complex depends on the metal, its oxidation state, and its ligands
Examples: [Cu(H2O)6]2+ pale blue; [Cu(NH3)6]2+ dark blue; d0 metal ions are
usually colorless.
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Crystal field theory was developed to explain the colors and
magnetism of coordination compounds
Major assumption: electrons on the metal ( d electrons) are repelled
by the ligand electrons
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dx2-y2, dz2
dxy, dxz, dyz
dxy, dxz, dyz
dx2-y2, dz2
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[Ti(H2O)6]3+
The spectrochemical series
Concept of high-spin and low-spin
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