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Transition Metals
Transition Metals
Chapter 22
Transition Metals
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Transition Metals: Occupy the d-block of periodic table
Have d-electrons in valence shell
Some characteristics of Transition Metals and their compounds:
1.
2.
3.
4.
5
5.
Exhibit more than one oxidation state
They form an extensive series of compounds known as metal complexes
l
or coordination compounds.
di ti
d
Transition metals exhibit interesting magnetic properties.
Many of their compounds are colored.
Transition metals play important roles in biological systems and Transition metals play important roles in biological systems
and
modern technology.
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There are periodic trends in the transition metals,
but they are often complex
(product of several factors, some working in opposite
directions – e.g.
e g combining the effects of increasing nuclear charge with
the presence of nonbonding d electrons)
Lanthanide contraction –
similarity in size, behavior, &
properties
p
p
of 4d and 5d
transition elements
We won’t worry about details of
periodic trends in the
transition metals or the exact
reasons for them
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Recall the Electron Configurations of the transition metals.
Ar
3s23p6
K
[Ar]4s1
Ca
[Ar]4s2
Sc
[Ar]3d14s2
Ti
[Ar]3d24s2
Use th
U
the periodic
i di table
t bl to
t gett electron
l t
configurations
fi
ti
of the First Row Transition metal ATOMS
Sc → Zn
V
Cr
Mn
Fe
Co
Ni
Cu
Zn [Ar] 3d104s2
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Be able to figure out the electron configuration of transition metal ions.
Transition Metals can have more than one oxidation state. Ti
[Ar]3d24s2
Ni
[Ar]3d84s2
Ti2+
Ti3+
Ni2+
Ti4+
Ti5+
Ni4+
Note: 4s is filled before 3d,
b t when
but
h oxidized,
idi d
4s electrons are lost before 3d.
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Some trends in Transition Metal Oxidation States
can be identified.
Oxidation States:
Highest oxidation states of Sc, Ti, V, Cr, Mn = number of valence
(4 + 3d) electrons.
(4s
l t
Sc [Ar]3d14s2
Mn [[Ar]3d
] 54s2
Trend from Sc → Mn:
The maximum oxidation state becomes increasingly unstable.
Sc3+, Ti4+
Are stable. Sc2O3 & TiO4 are stable oxides.
7+
Mn
E ists b
Exists
butt is easil
easily red
reduced.
ced
−
MnO4
Strong oxidizing agent.
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Many transition metals form compounds that have fun colors!
– colors are due to oxidation state and electron g
p
configuration...more specifics about that later!
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Transition metals form COMPLEXES.
Transition metal ions are Lewis acids →
Ligands are Lewis bases →
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Metal complex is formed when a metal is bonded to molecules or ions.
Complex ion: metal complex that is not neutral
Coordination compounds: compounds that contain complexes
Coordination number: # of atoms directly bonded to a metal
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The coordination of the ligand with the metal can greatly alter its physical or chemical properties EXAMPLE: color Transition Metals
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Chemical properties of Metal Complexes differ from the properties of the metal alone
the properties of the metal alone. For example: the ease of oxidation of the metal changes when a metal complex forms
metal complex forms. Which is easier to reduce, the metal ion or the complex?
Ag+(aq) + e− → Ag(s)
E°1/2= +0.80V
[Ag(CN)2]−(aq) + e− → Ag(s)+ 2CN−(aq) E°1/2 = −0.31V
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Complexes are characterized by their Oxidation State and Coordination Number, but the identity of the ligands also matters. • Oxidation State: “primary valence”
• Coordination Number: “secondary valence”
Early Formula Color
# of Ions per Free Cl− Ions
formula Unit in Formula
Complex ion Formula
CoCl3 6NH3
orange
4
3
[Co(NH3)6]Cl3
CoCl3 5NH3
purple
3
2
[Co(NH3)5Cl]Cl2
CoCl3 4NH3
green
2
1
trans‐[Co(NH3)4Cl2]Cl
CoCl3 4NH3
violet
2
1
cis‐[Co(NH3)4Cl2]Cl
The four compounds in the table below have the same oxidation state and coordination number, but clearly they have different properties. Transition Metals
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Coordination Sphere is the central atom + ligands bonded to it
Use [ ] to set off components in the coordination sphere
[]
p
p
Early Formula Color
# of Ions per Free Cl− Ions
formula Unit in Formula
in Formula
Complex ion Formula
CoCl3 6NH3
orange
4
3
[Co(NH3)6]Cl3
CoCl3 5NH3
purple
3
2
[Co(NH3)5Cl]Cl2
CoCll3 4NH3
green
2
1
trans‐[Co(NH
[ ( 3)4Cll2]Cl
] l
CoCl3 4NH3
violet
2
1
cis‐[Co(NH3)4Cl2]Cl
What happens to these complexes when dissolved in water?
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Knowing the charge on a complex ion and the charge on each ligand, one can determine the oxidation number for the metal
one can determine the oxidation number for the metal.
[[Cu(NH
( 3)4]]SO4
K2[[Ni(CN)
( )4]] Charge on the complex:
Charge on the complex:
Coordination #:
Coordination #: Coordination #: Oxidation state of the metal:
Oxidation state of the metal:
Oxidation state of the metal:
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Or, knowing the oxidation number on the metal and the charges on the ligands, one can calculate the charge on the complex ion.
Fe3+ surrounded by six water molecules
Pt(II) surrounded by two ammonia molecules and two bromide ions
molecules and two bromide ions
Charge on the complex:
Charge on the complex:
Coordination number:
Coordination number:
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Geometries of Transition Metal Complexes
Geometry for Coordination # = 4
• Tetrahedral
• Square Planar
Square Planar
[Ni(CN)4]22
[Zn(NH3)4]2+
[PtCl2(NH3)2]
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Geometries of Transition Metal Complexes
Geometryy for Coordination # = 5
• Trigonal Bipyramidal
[Fe(CO)5]
[Re(SCH2C6H4OCH3-p)3(PPh3)2]
ReL3(PR3)2
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Geometries of Transition Metal Complexes
Geometryy for Coordination # = 6
• Octahedral
chromium(III) and cobalt(III) consistently have h
i (III) d b lt(III)
i t tl h
the same coordination number of 6
3
[CoF6]3−
[Co(en)3]3+
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Special Ligands: Chelating Agents
Chelates are ligands possessing two or more donor atoms.
atoms
• Mono‐dentate Ligands
Examples:
[Cu(H2O)2(NH3)2]2+
• Bi‐dentate Ligands
Example: ethylenediamine = en
:NH2−CH2−CH2− :NH2
[Cu(H2O)2(en)]2+
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These are examples of Bi-dentate Ligands
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Poly‐dentate ligands bind to the metal in more than 2 sites.
Example: ethylenediaminetetraacetic acid (EDTA)
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Below are representations of metal complexes with ethylenediamine and EDTA as the ligands. Transition Metals
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EDTA is used to sequester metal ions.
ethylenediaminetetraacetic acid (EDTA)
Applications:
Applications: Removes trace metals ions that catalyze food decomposition
Used in poison control
Used by lichen to obtain
y
minerals from rock.
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Chelating Agents can be used to soften water
Used in shampoo to remove Used
in shampoo to remove
trace metals from hard water (Ca2+ and Mg2+): EDTA
Na5P3O10 Used in detergents to remove trace amounts of
dissolved metals:
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Chelating Agents play important roles in biological applications. applications
Porphine is a flat molecule
Complexes with this molecule
Complexes with this molecule are called porphyrins
CH2
CH3
CH
CH2CH3
H3C
N
N
Mg
N
N
CH3
H3C
C20H39OOCH2CH2C
H3COOC
O
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These are the Important Chelating Agents you need to know.
Chelate
# of Coordination
Sites
Charge
Ethylenediamine (en)
P
Porphine
hi
EDTA
Oxalate (C2O42−)
Carbonate (CO322−)
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METAL COMPLEXES are very stable.
The formation reactions have large values of K
g
f. Cu(OH2)42+ + 4NH3 → Cu(NH3)42+ + 4H2O
Cu2+(aq) + 4NH3 → Cu(NH3)42+ + 4H2O
Kf VALUES OF SOME COMPLEXES
[Ag(NH3)2] +
[Cu(NH3)4] 2+
[Cu(CN)4 ] 2−
[Ag(CN)2 ] −
[Ag(S2O3)2 ] 3−
1.7 × 107
5 × 1012
1 × 1025
1 × 1021
2.9 × 1013
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THE CHELATE EFFECT: Chelating ligands form exceptionally stable metal complexes when compared to related monodentate ligands.
[Ni(H2O)6]2+ + 6 NH3
[Ni(H2O)6]2+ + 3 en
[Ni(NH3)6]2+ + 3 en
[Ni(NH3)6]2+ + 6H2O
[Ni(en)3]2+ + 6H2O
Kf = 4×108
Kf = 2×1018
[Ni(en)3]2+ + 6 NH3
What is the expression for the formation constant for this reaction?
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THE CHELATE EFFECT: Chelating ligands form exceptionally THE CHELATE EFFECT
stable metal complexes when compared to related monodentate
d t t ligands.
li d
This “chelate effect” is due to :
1)
Probability
2)
Entropy Effects
Probability Effect:
Cd+2
H2
N
NH2CH3
Cd+2
NH2
NH2CH3
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ΔS for the formation of a chelate is positive.
1) Cd
1)
Cd2+ + 4CH
4CH3NH2
[Cd(CH3NH2)4]2+
2) Cd2+ + 2en
1
ΔH° = −57.3 kJ
57 3 kJ
[Cd(en)2]2+ + 4H2O
ΔH° = −56.5 kJ
Ligand
ΔH°(kJ)
Methyl amine
y
CH3NH2
−57.3
−37.2kJ
−56.5
−60.7kJ
2
ΔS°(J/K)
ΔG°
Why is ΔS° so much larger?
[Cd(H2O)4]2+ + 4 CH3NH2
[Cd(CH3NH2)4]2+ + 4H2O
[Cd(H2O)4]2+ + 2 en
Transition Metals
[Cd(en)2]2+ + 4H2O
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