Download All contain a transition metal!! All are coordination compounds Many

CHEM 1311A Syllabus
• Transition metals and Coordination Chemistry
– Introduction to coordination compounds; stereochemistry,
isomerism and nomenclature
– Coordination compounds: bonding models and energetics
– Coordination compounds: equilibria and substitution reactions
• Bioinorganic chemistry
Fourth Exam – Friday, November 30
What do these have in common?
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Hemoglobin
Myoglobin
Automobile paints
Anti-cancer drugs (some)
Industrial catalysts (many)
Arthritis drugs
Vitamin B12
Cytochromes
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“Blue blood”
Ferredoxins
Rubies
Emeralds
Legumes (nitrogen fixers)
Radiopharmaceuticals (some)
MRI contrast agents
All contain a transition metal!!
All are coordination compounds
Many are colored
1
Transition metal complex (coordination
compound) terminology
• Coordination compound, coordination complex, complex - a
•
•
•
compound containing a metal ion and appended groups, which are
Lewis bases and may be monatomic or polyatomic, neutral or
anionic.
Ligand - Lewis base bonded (coordinated) to a metal ion in a
coordination complex.
–Those with only one point of attachment are monodentate
ligands.
–Ligands that can be bonded to the metal through more than one
donor atom are termed bidentate (two points of attachment),
tridentate, etc. Such ligands are termed chelating ligands.
Coordination number - number of ligands coordinated to a metal
ion, 2-12.
Coordination geometry or stereochemistry (octahedral,
tetrahedral, square planar) - geometrical arrangements of ligands
(donor groups) about a metal ion.
Effect of coordination number and geometry
on absorption spectrum
Comparison of electronic absorption
spectral intensities for [Co(OH2)6]2+
(octahedral) and [CoCl4]2- (tetrahedral)
2
Transition metal complex (coordination
compound) terminology
• Isomers
– Constitutional (structural) isomer - one of two or more
compounds having the same composition but differing in their
atom connectivities.
– Stereoisomer - one of two or more compounds having the
same atom connectivities but different spatial arrangements of
atoms.
○ Diastereoisomer – stereoisomers not related by mirror
images
○ Enantiomer - one of a pair of species that are nonsuperimposable mirror images.
Types of Isomerism
Constitutional
(structural)
Stereo
Linkage
Diastereomers
(geometric)
Ionization
Enantiomers
(optical)
Hydration
● Constitutional (structural) isomers – same composition, different
atom connectivities
● Stereoisomers – same composition, same atom connectivities,
different spatial arrangements
3
Stereoisomers: Diastereoisomers
• Compounds that have the same atom connectivities, but
which are not mirror images are diastereoisomers.
X
M
L
L
L
L
X
M
cis
X
X
L
X
X
M
trans
L
square planar
tetrahedral
Stereoisomers: Diastereoisomers
• Compounds that have the same atom connectivities, but
which are not mirror images are diastereoisomers.
L
ML4X2
L
L
M
L
X
L
X
L
X
L
M
L
L
L
M
L
cis
M
L
trans
L
X
L
X
X
X
L
X
L
X
L
L
L
M
L
X
L
L
X
M
X
L
trans
L
4
Stereoisomers: Diastereoisomers
• Compounds that have the same atom connectivities, but
which are not mirror images are diastereoisomers.
ML3X3
X
L
L
M
X
L
L
X
L
X
L
M
L
L
mer
X
X
X
L
M
X
X
L
X
L
M
X
fac
X
L
Stereoisomers: Enantiomers
• Compounds that have no center or plane of symmetry exist
in non-superimposable, mirror-image forms.
Rotate by 180E
F
F
C
Br
H
Cl
Cl
C
Br
H
5
Stereoisomers: Enantiomers
• Compounds that have no center or plane of symmetry exist
in non-superimposable, mirror-image forms.
H2
N
H2N
Co
N
H2 H N
2
NH2
3+
NH2
How many diastereoisomers can exist for the complex ion
[Co(H2NCH2CH2NH2)(NH3)2Cl2]+ ?
How many of these diastereoisomers have nonsuperimposable mirror
image forms?
6
How many diastereoisomers can exist for [Co(dien)(Cl)(NO2)2]?
dien =
H2N
NH2
NH
H
N
=
NH2
H2N
How many of the diastereoisomers that can exist for
[Co(dien)(Cl)(NO2)2] have non-superimposable mirror images, i.e., are
enantiomeric?
N
N
N
O2N
Co
Cl
NO2
N
N
Cl
Co
NO2
N
NO2
N
N
N
Co
NO2
N
Cl
N
NO2
N
Co
Cl
NO2
NO2
7
How many stereoisomers (diastereoisomers and enantiomeric forms)
can exist for [Co(H2NCH2CH2O)3]?
The tetradentate ligand shown below forms six-coordinate complexes
with Co(III) having the composition [CoLX2]+ where X is a mondentate
ligand.
HN
NH2
N
N
N
N
=
HN
NH2
How many diastereoisomers can be formed? How many are
enantiomeric?
8
Energy changes for formation of ML6n+
n+
M +6L
n+
n+
ML6
ML6
(octahedral)
E
d z 2 d x2 - y 2
e-e replusion
d xz dxy dyz
)E
differential replusions
of d orbitals
electrostatic
attraction
Magnetic properties
• High spin – maximum number of unpaired electrons for dn
– Spin pairing energy is greater than ∆E (∆o)
• Low spin – minimum number of unpaired electrons for dn
– Spin pairing energy is less than ∆E (∆o)
9
Dependence of magnetic and spectral
properties on ligand type
• Spectrochemical series
– I– < Br– < Cl– < SCN– < F–, OH– < NO2– < H2O < SCN– < NH3 <
en < NO2– < PR3 < P(OR)3, C2H4< PF3, CO, CN–
Energy level diagram for complex
with F donor ligands
F*
np
F*
ns
eg *
)
(n-1) d
t2g
F*
n
L orbitals
F
F
F
10
Metal-ligand B-bonding interactions
dB-pB donor interactions; halide, hydroxide
dB-pB acceptor interactions (rare)
dB-dB acceptor interactions; phosphorus, arsenic
dB-B* acceptor interactions; CO, CN-, NO, RNC
dB-B* acceptor interactions; olefins (C=C)
Energy level diagram for complex
with F and B donor ligands
F*
np
F*
eg *
ns
) t
2g
F*
B*
(n-1) d
n
B
F
F
F
L orbitals
11
Comparison of level diagrams for complexes
with F only and F plus B donor ligands
np
ns
)
)
(n-1) d
L orbitals
Energy level diagram for complex
with F and B acceptor ligands
F*
np
ns
F*
B*
n
eg *
F*
L pi acceptor
orbitals
)
(n-1) d
t2g
B
L orbitals
F
F
F
12
Comparison of level diagrams for complexes
with F only and F plus B acceptor ligands
np
L pi acceptor
orbitals
ns
)
)
(n-1) d
L orbitals
Energy level diagrams for complexes with F only,
F plus B donor, and F plus B acceptor ligands
np
L pi acceptor
orbitals
ns
)
)
)
(n-1) d
L orbitals
13
HOMO and LUMO for cyanide ion
p
p
E
s
s
Effect of B-donor and B-acceptor
interactions on ) in octahedral complexes
eg *
eg *
)
eg *
)
t2g
)
t2g
t2g
energy of d-orbitals
prior to interaction
with ligands
F bonding only
F + B donor
intermediate
field ligands
weak field
ligands
F + B acceptor
strong field
ligands
14
Dependence of magnetic and spectral
properties on ligand type
• Spectrochemical series
– I– < Br– < Cl– < SCN– < F–, OH– < NO2– < H2O < SCN– < NH3 <
en < NO2– < PR3 < P(OR)3, C2H4< PF3, CO, CN–
• Strong field ligands = low-spin complexes
– have B-acceptor orbitals: B* as in CO or CN–, B*as in
CH2=CH2, low lying d as in PR3, PF3
• Weak field ligands = high-spin complexes
– have B-donor orbitals: usually multiple p orbitals as in X
• Intermediate field ligands = usually high spin for +2 ions,
low-spin for +3 ions
– have few, or no, B -donor or acceptor orbitals, or there is a
poor match in energy of available B orbitals: NH3, H2O, pyridine
Variation of )O in octahedral Ti(III)
complexes
Ti(III) is a d1 ion and exhibits one absorption in the electronic
spectrum of its metal complexes due to transition of the electron
from the t2g (lower energy) orbitals to the eg (higher energy)
orbitals. The energy of the absorption corresponds to )O.
Ligand
BrCl(H2N)2C=O
NCSFH2O
CN*E = h< = hc/8
)O/cm-1*
11,400
13,000
17,550
18,400
18,900
20,100
22,300
15
Electronic absorption spectra
• Selection rules
– Transitions that occur without change in number of
unpaired electrons (spin multiplicity) are allowed
– Transitions that involve a change in the number of
•
•
unpaired spins are “forbidden” and are therefore of low
intensity.
> solutions of high-spin d5, e.g., Mn(II), complexes are
lightly colored
Absorption bands are broad because metal-ligand bonds are
constantly changing distance (vibration) and since electronic
transitions occur faster than atomic motions this means that
there are effectively many values of ∆o.
d1 and d9, and high-spin d4 and d6 ions have only one spinallowed transition; high-spin d2, d3, d7 and d8 have three spinallowed transitions
Allowed vs forbidden transitions
dx2-y2 dz2
E
dxz
dxy
dyz
dx2-y2 dz2
dxz
dxy
dyz
dx2-y2 dz2
dxz
dxy
dyz
dx2-y2 dz2
dxz
dxy
dyz
16
Effect of ligand on absorption
spectra (and color)
Number of d electrons and spectral intensity
[Mn(OH2)6]2+
17
Transitions in d1 and d2 complexes
dx2-y2 dz2
dxz
dxy
dyz
dx2-y2 dz2
dx2-y2 dz2
E
dx2-y2 dz2
dxz
dxy
dxz
dyz
dxz
dx2-y2 dz2
dxz
dxy
dyz
dxy
dxy
dyz
dxy1 dx2-y21 dxz1dz21 dyz1dz21
dxz1 dx2-y21 dxy1dz21 dyz1dx2-y21
dyz
dx2-y2 dz2
dxz
dxy dyz
Comparison of crystal field splittings for
octahedral, square planar and tetrahedral
ligand fields
18
Crystal field splitting in tetrahedral complexes
•
•
•
Tetrahedral arrangement of four ligands
showing their orientation relative to the
Cartesian axes and the dyz orbital.
The orientation with respect to dxz and dxy is
identical and the interaction with these
orbitals is considerably greater than with the
dz2 and dx2- y2 orbitals; therefore the dyz, dxz
and dxy orbitals are higher in energy than
dz2 and dx2- y2 .
Because there are only four ligands and
the ligand electron pairs do not point
directly at the orbitals, ∆t ~4/9 ∆o. As a
result the spin-pairing energy is always
greater than ∆ and tetrahedral complexes
are always high spin.
Factors affecting the magnitude of )
(Crystal Field Splitting)
• Charge on the metal. For first row transition elements )O varies
from about 7,500 cm-1 to 12,500 cm-1 for divalent ions and 14,000
cm-1 to 25,000 cm-1 for trivalent ions.
• Position in a group. )O values for analogous complexes of
metal ions in a group increase by 25% to 50% on going from one
transition series to the next. This is illustrated by the complexes
[M(NH3)6]3+ where ) values are 23,000 cm-1 for M=Co; 34,000 cm1 for M=Rh and 41,000 cm-1 for M=Ir.
• Geometry and coordination number. For similar ligands )t will
be about 4/9 )O. This is a result of the reduced number of ligands
and their orientation relative to the d orbitals. Recall that the
energy ordering of the orbitals is reversed in tetrahedral
complexes relative to that in the octahedral case.
• Identity of the ligand. The dependence of ) on the nature of the
ligand follows a regular order, known as the spectrochemical
series, for all metals in all oxidation states and geometries.
19
Thermodynamic vs kinetic stability
• Stability in a thermodynamic sense refers to the energetics of a
•
•
•
•
formation or decomposition reaction )G = )H + T)S
Stability in a reactivity sense refers to the rate with which a
given reaction occurs.
Complexes that undergo substitution with half-lives less than
about one minute are referred to as labile; those that are less
reactive are termed inert.
Complex stability and reactivity do not necessarily correlate with
ligand field strength; the latter refers to spectroscopic and
magnetic properties.
Thermodynamic and kinetic stabilities sometimes parallel but
often they do not.
– [Ni(CN)4]2& illustrates the latter case; the overall equilibrium
constant its formation is >1030 but the second order rate
constant for CN& exchange is >5 x 105 M-1 s-1
Stepwise formation of [Cu(NH3)4]2+
[Cu(OH2)4]2+ + NH3 W [Cu(OH2)3(NH3)]2+ + H2O
[Cu(OH2)3(NH3)]2+
[Cu(OH2)2(NH3)2
[Cu(OH2)(NH3)3
]2+
]2+
+ NH3 W
[Cu(OH2)2(NH3)2]2+
+ NH3 W
[Cu(OH2)(NH3)3]2+
+ NH3 W [Cu(NH3)4
]2+
log K1 = 4.22
+ H2O
log K2 = 3.50
+ H2O
log K3 = 2.92
+ H2O
[Cu(OH2)4]2+ + 4 NH3 W [Cu(NH3)4]2+ + 4 H2O
log K4 = 2.18
[Cu(NH ) 2+ ]
34
4 [Cu2+ ][NH ]4
3
β =
20
Speciation is determined by ligand concentration
[Cu(OH2)4]
2+
2+
+ n NH3 = [Cu(OH2)4-n(NH3)n]
1.0
0.9
0.8
n=0
Fraction
0.7
0.6
n=1
0.5
n=4
n=2
n=3
0.4
0.3
0.2
0.1
0.0
6
4
2
0
-log[NH3]
The Chelate Effect is largely entropic in origin
[Cu(OH2)4]2+ + en W [Cu(OH2)2(en)]2+ + 2 H2O
)H = -54 kJ
mol-1,
log K1 = 10.6
)S = 23 J K-1 mol-1
[Cu(OH2)4]2+ + 2 NH3 W [Cu(OH2)2(NH3)2]2+ + 2H2O
)H = -46 kJ
mol-1,
log K2 = 7.7
)S = -8.4 J K-1 mol -1
21
Ligand substitution in coordination complexes
• Arguably the most important reaction of coordination complexes
•
•
is ligand substitution.
There are two limiting mechanisms for substitution reactions
– associative parallels the SN2 reaction in organic chemistry;
the reaction involves an intermediate of higher
coordination number. rate = k[complex][L]
> associative reactions are more important for larger metal
ions and for those that have vacancies in the t2g orbitals
– dissociative parallels the SN1 reaction in organic chemistry;
the reaction involves an intermediate of lower
coordination number. rate = k[complex]
The simplest substitution is ligand exchange which is not
complicated by thermodynamics since ∆G = 0.
– exchange rates of water have been most extensively studied
– rate constants for water exchange range from 1.1x10-10 s-1 to
5x109 s-1
Observations on water exchange
• An increase in oxidation state for the metal reduces the rate of
•
•
•
exchange
Early (larger) elements in a period tend to have a greater
contribution from associative processes
Heavier (larger) elements in a family have a greater contribution
from associative processes; also greater bond strengths
decrease rate of dissociative processes
Occupancy of (antibonding) eg orbitals increases the rate for all
oxidation states
22
Water exchange rates in aquo metal ions
23
Rate constantsa for water exchange
n+
[MLnn(OH2)]n+
k/s-1-1
n+
[MLnn(OH22)]n+
k/s-1
[Ti(OH2) 6]3+
1.8 x 105
[V(OH2)6] 2+
8.7 x 101
[V(OH2)6] 3+
5.0 x 102
[Cr(OH2)6] 2+
>108
[Cr(OH2)6] 3+
2.4 x 10-6
[Mn(OH2)6] 2+
2.1 x 107
[Fe(OH2)6]2+
4.4 x 106
[Fe(OH2 )6]3+
1.2 x 102
[Ru(OH ) ]
1.8 x 10
-2
[Ru(OH ) ]
3.5 x 10-6
[Co(OH ) ]
3.2 x 10
6
[Ni(OH2)6] 2+
3.2 x 104
[Pd(OH2) 4]
5.6 x 10-2
[Pt(OH2)4]2+
3.9 x 10-4
[Cu(OH2)6]2+
>107
[Zn(OH2)6]2+
>107
[Cr(NH3)5OH2]3+
5.2 x 10-5
[Co(NH3) 5OH2]3+
5.7 x 10-6
[Rh(NH3) 5OH2]3+
8.4 x 10-6
[Ir(NH3)5OH2]3+
6.1 x 10-8
2 6 2+
2 6 2+
2+
2 6 3+
aAll
rate constants are expressed as first order rate constants for
comparative purposes even though some reactions are associative.
Electron transfer reactions: importance of
orbital occupancy and spin state on rate
•
Electron transfer is second only to substitution in importance as a
characteristic reaction of coordination complexes and especially in
biological systems.
•
Again the simplest reaction is outer-sphere electron exchange where
)G=0
•
Rates of electron exchange vary enormously across the transition series,
but two things are invariably true:
– The rate of electron transfer is greatest when electrons are transferred
from a t2g orbital on the reductant to a t2g orbital on the oxidant.
– There is minimal change in bond distance in either oxidant or reductant
upon electron transfer.
e- config
M-L BD, D
kex, M-1 s-1
[Co(NH3)6]2+
t2g5eg2
2.114
# 10-9
[Co(NH3)6]3+
t2g6
1.936
[Ru(NH3)6]2+
t2g
6
2.144
t2g
5
2.104
Complex
[Ru(NH3)6]3+
8.2 x 102
24