Download transition metal complexes

Transition Metals
•!
•!
Occupy the d-block of periodic table
Have d-electrons in valence shell
Characteristics:
1.! More than one oxidation state
2.! Many compounds are colored
3.! Interesting magnetic properties
4.! Form Metal complexes or Coordination compounds
5.! Transition metals play important roles in biological
systems and modern technology.
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1!
CHEM112 LRSVDS Transition Metals part 1
Electron Configurations and Oxidation States
Many transition metals form compounds that have fun colors!
– colors are due to oxidation state and electron
configuration...more specifics about that later!
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Periodic trends
There are periodic trends in the transition metals, but they
are often complex
(product of several factors, some working in opposite
directions – e.g. combining the effects of increasing nuclear
charge with the presence of nonbonding d electrons)
Lanthanide contraction – similarity
in
size, behavior, properties 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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Electron Configuration: Order of Orbital Filling
CHEM112 LRSVDS Transition Metals part 1!
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VALENCE ELECTRON CONFIGURATIONS
Orbital Filling in First Row:
Sc ! Zn
[Ar] = 3s23p6
K [Ar] 4s1
Ca [Ar] 4s2
Sc [Ar] 3d14s2
Ti [Ar] 3d24s2
.
.
.
.
.
.
Zn [Ar] 3d104s2
*Note: 4s is filled before 3d,
but when metal is oxidized, 4s electrons are lost before 3d.
Ti
Ti2+
Ti3+
Ti4+
Ti5+
[Ar]3d24s2
[Ar]3d24s0
[Ar]3d14s0
[Ar]3d04s0
does not exist!
CHEM112 LRSVDS Transition Metals part 1!
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Transition Metal Oxidation States
Group 3B – 7B:
For Sc, Ti, V, Cr, Mn: highest oxidation states common
Highest oxidation states = number of valence (4s + 3d) electrons.
Sc [Ar]3d14s2
Mn [Ar]3d54s2
Sc3+ [Ar] (maximum oxidation state is +3)
Mn7+ [Ar] (maximum oxidation state is +7)
Group 8B, 1B, 2B:
The maximum oxidation state becomes increasingly unstable and uncommon.
Sc3+
Mn7+
Fe8+
Sc2O3 is a stable oxide
Exists but is easily reduced (MnO4! strong oxidizing agent)
Does not exist; unstable
CHEM112 LRSVDS Transition Metals part 1!
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Metal Complexes or
Coordination Compounds
Transition metal ions are Lewis acids "
they accept electron pairs
Ligands are Lewis bases "
molecules or ions which donate electron pairs
CHEM112 LRSVDS Transition Metals part 1!
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Ligands: Donate Lone Pairs of Electrons
F!,
Cl!,
Anionic Ligands
CN!, SCN!, NO2!, EDTA4!
Br!,
Neutral Ligands
NH3, H2O, CO, CH3OH, en
Mono-dentate Ligands:
Only ONE donor atom is bound to the metal (single tooth to hold onto metal d orbital)
Examples;
NH3,
H-O-H ,
CH3-O-H
Bi-dentate Ligands:
TWO donor atoms are bound to the metal (has 2 teeth to hold onto metal d orbitals).
Example; H2N-CH2-CH2-NH2 (en or ethylenediamine)
Poly-dentate Ligands
Have more than one functional group with lone pairs (many teeth!)
Example; EDTA (ethylenediaminetetraacetic acid)
CHEM112 LRSVDS Transition Metals part 1!
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Coordination Compounds
[Cu(H2O)2(NH3)2]2+
[Cu(H2O)2(en)2]2+
en =!
Coordination number:
number of donor atoms attached to the metal.
Chelates: “chele or Claw”
ligands possessing two or more donor atoms.!
CHEM112 LRSVDS Transition Metals part 1!
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TRANSITION METAL COMPLEXES
[Cu(NH3)4)]SO4
SO42- + [Cu(NH3)4)]2+
Charge on the complex:
Coordination #:
Oxidation state of the metal:
K2[Ni(CN)4)]
2 K+ + [Ni(CN)4]2-
Charge on the complex:
Coordination #:
Oxidation state of the metal:
CHEM112 LRSVDS Transition Metals part 1!
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Werner’s Theory
•! This approach correctly
predicts there would be two
forms of CoCl3 · 4 NH3.
–! The formula would be written [Co
(NH3)4Cl2]Cl.
–! One of the two forms has the two
chlorines next to each other.
–! The other has the chlorines
opposite each other.
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Geometry of Transition Metal Complexes:
Coordination Number of 4
Tetrahedral
e.g. [Zn(NH3)4]2+
Square Planar
e.g. [Ni(CN)4]2-
e.g. [PtCl2(NH3)2]
Cl
Pt
Cl
CHEM112 LRSVDS Transition Metals part 1!
NH3
NH3
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Geometries of Transition Metal Complexes
Geometry for Coordination # = 5
•! Trigonal Bipyramidal
[Re(SCH2C6H4OCH3-p)3(PPh3)2]
ReL3(PR3)2!
[Fe(CO)5]
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Six Coordinate Complexes:
Octahedral (six vertices, eight FACES)
e.g. [CoF6]3F
F
F
Co
F
F
F
e.g. [Co(en)3]3+!
N
N
N
Co
N
N
N
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Poly-dentate Ligands: Form Metal CHELATES!
ethylenediamine (“en”)!
= ethylenediaminetetraacetate ion!
!
CHEM112 LRSVDS Transition Metals part 1!
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IMPORTANT CHELATING LIGANDS
O
HOCCH2
O
NCH2CH2N
:
HOCCH2
:
EDTA
CH2COH
CH2COH
O
O
CHEM112 LRSVDS Transition Metals part 1!
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Uses of Chelating Agents
•! Used to “sequester” or “seize” metal ions
•! Used in detergents to remove trace amounts of
dissolved metals: Na5P3O10
•! Complex trace metal ions that catalyze food
decomposition: EDTA
•! Used in poison control: EDTA
•! Used in shampoo and cleaning products to
remove trace metals from hard water (Ca2+ and
Mg2+): EDTA
!
CHEM112 LRSVDS Transition Metals part 1!
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IMPORTANT CHELATING LIGANDS
Porphine forms metal complexes called Phorphyrins
+ Fe + protein ! !
!"#$%#&'()
Fe!
CHEM112 LRSVDS Transition Metals part 1!
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Chemistry and Life!
!"#$%$&"'##()!
http://fr.academic.ru/dic.nsf/frwiki/29449!
*+%%,-"%$.+!
http://en.wikipedia.org/wiki/File:Ferrichrome.png!
!
CHEM112 LRSVDS Transition Metals part 1!
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Important Chelating Agents
Chelate
# of Coordination
Sites
Charge
Ethylenediamine
Porphine
EDTA4Oxalate (C2O42-)
Carbonate (CO32-)
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Metal Complexes and Isomers
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Structural Isomers
•! Coordination Sphere
Isomers
–! If a ligand (like the NO2
group at the bottom of the
complex) can bind to the
metal with one or another
atom as the donor atom
•! Linkage Isomers
–! differ in what ligands are
bonded to the metal and
what is outside the
coordination sphere
Three isomers of CrCl3(H2O)6 are:
violet [Cr(H2O) 6]Cl3
green [Cr(H2O) 5Cl]Cl2·H2O
green [Cr(H2O) 4Cl2]Cl·H2O
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Stereoisomers
•! Geometric Isomers
•! Optical Isomers
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METAL COMPLEX STABILITY
Cu(OH2)42+ + 4NH3
[Cu(NH3)4]2+ + 4H2O(l)
Cu2+(aq) + 4NH3
[Cu(NH3)4]2+ + 4H2O(l)
Kf = !
[H2O] = constant
Formation Constant: Kf is very large!
[Ag(NH3)2]+
[Cu(NH3)4]2+
[Ag(S2O3)2]3!
[Ag(CN)2] !
[Cu(CN)4]2!
1.7 x 107
5.0 x 1012
2.9 x 1013
1.0 x 1021
1.0 x 1025
CHEM112 LRSVDS Transition Metals part 1!
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THE CHELATE EFFECT
Chelating ligands form exceptionally stable metal complexes.
[Ni(H2O)6]2+ + 6NH3
[Ni(NH3)6]2+ + 6H2O
Kf = 4x108
[Ni(H2O)6]2+ + 3en
[Ni(en)3]2+ + 6H2O
Kf = 2x1018
*DUE TO:
1) PROBABILITY
2) ENTROPY EFFECTS!
Probability Effect:!
Cd+2
H2
N
NH2CH3
Cd+2
NH2
NH2CH3
!
CHEM112 LRSVDS Transition Metals part 1!
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Entropy and the Chelate Effect
1) Cd2+ + 4CH3NH2
2) Cd2+ + 2en
[Cd(CH3NH2)4]2+
[Cd(en)2]2+!
Ligand
#H°(kJ)
#S°(J/K)
#G°
1
methyl amine
-37.2kJ
2
en
-60.7kJ
Why is #S° so much larger?
[Cd(H2O)4]2+ + 4CH3NH2
[Cd(H2O)4]2+ + 2en
[Cd(CH3NH2)4]2+ + 4H2O
[Cd(en)2]2+ + 4H2O
CHEM112 LRSVDS Transition Metals part 1!
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Magnetic Properties of Transition Metals
Diamagnetic:
unaffected by a magnetic field
Paramagnetic:
influenced by a magnetic field
The magnetic properties depend on the number of unpaired electrons
Na+
Mn2+
Ti2+
Co3+
CHEM112 LRSVDS Transition Metals part 1!
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Magnetic Behavior
# unpaired e" ! prior alignment
alignment in magnetic field
!attracted to magnetic field !
Diamagnetic!
Paramagnetic!
Ferromagnetic!
CHEM112 LRSVDS Transition Metals part 1!
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CRYSTAL FIELD THEORY
The presence of metal-ligand bonding electrons raises the energy
of metal d orbitals due to electrostatic repulsion.
E
d orbitals in uniform,
“spherical” field of negative
charge; all orbitals raised in
energy equally
d orbitals in free
metal ion (all
degenerate)!
CHEM112 LRSVDS Transition Metals part 1!
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Octahedral Metal Complexes; Effect of metalligand bonding electrons on metal d orbitals
Which metal d orbitals are most affected by the metalligand bonding electrons?!
Metal ion in Octahedral charge Field
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CRYSTAL FIELD SPLITTING
Crystal Field Energy Splitting of d orbitals in octahedral ligand field!
e set (dz2, dx2–y2)
E
#$ “delta octahedral”
t2 set (dxy, dxz, dyz)!
d orbitals in uniform,
“spherical” field of
negative charge!
d orbitals in octahedral
field of negative charge!
#!o= Crystal field splitting energy
!o depends on:
1.
2.
3.
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Spin Pairing Energy
P = spin pairing energy
The energy required to place two electrons of opposite
spin in the same orbital
*magnitude of P is independent of the ligands
If # is large (P < #) " Low Spin Complex
If # is small (P > #) " High Spin Complex
For a transition metal with 5 d electrons;!
First 3 electrons fill lower E orbitals!
CHEM112 LRSVDS Transition Metals part 1!
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Cl! <
SPECTROCHEMICAL SERIES:
Ability of L to increase the energy gap
F! < H2O < NH3 < en < NO2! <
CN!
Increasing # %
CN!
CO
NO2!
en
NH3
H 2O
Oxalate
OHF!
SCN!
Cl!
BrI!
Strong field!
ligands!
Weak field !
ligands!
absorbs!
observed!
CHEM112 LRSVDS Transition Metals part 1!
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MAGNETIC PROPERTIES
Which diagram corresponds to CoF63- and
which corresponds to Co(CN)63- and why? !
E
High spin
Paramagnetic!
Low spin (spin-paired)
Diamagnetic
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Practice Problems
1. Ammonia is a strong field ligand. Is [Mn(NH3)6]3+ high spin or low spin?
2. The oxalate complex [Co(C2O4)3]4& has 3 unpaired electrons. Is it high
spin or low spin?
CHEM112 LRSVDS Transition Metals part 1!
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OPTICAL PROPERTIES; COLOR OF COMPLEXES
Observed color is related to the amount of
energy required to promote an electron.
Compare # to energy absorbed.
CHEM112 LRSVDS Transition Metals part 1!
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OPTICAL PROPERTIES OF TRANSITION METALS
Color depends on identity of the ligands
[Ni(H2O)6]2+ + 6 NH3 ! [Ni(NH3)6]2+ + 6 H2O
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COLOR ABSORPTION BY METAL COMPLEXES
When light of a certain wavelength is absorbed by a complex,
the complex will appear the complementary color of the
wavelength absorbed
Observed Color:
1)
OR
2)
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Visible Absorption Spectra:!
What color is Ti(H2O)63+?!
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Color of Metal Complexes
SCN"!
[Fe(H2O)6]3+!
[Fe(SCN)(H2O)5]2+!
1. Which of these complexes absorbs light at a shorter wavelength?
2. Which complex has the larger #o?
3.! Which is a weaker field ligand; water or thiocyanate?
4. What color will TiO2 be? What color will ZnO be?
CHEM112 LRSVDS Transition Metals part 1!
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Reduction Potential of Metal Complexes
Stability of Transition Metal Complexes depends on
reduction potential of the metal complex.
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
CHEM112 LRSVDS Transition Metals part 1!
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USES OF TRANSITION METALS
Titanium
Vanadium
Chromium
Manganese
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