Download Crystal Field Theory

• Focus:
Crystal Field Theory
energies of the d orbitals
• Assumptions
• 1. Ligands: negative point charges
• 2. Metal-ligand bonding: entirely ionic
• strong-field (low-spin): large splitting of d
orbitals
• weak-field (high-spin): small splitting of d
orbitals
d-orbital energy level
diagram for tetrahedral
dxy dxz dyz
_ _ _

_ _
E
isolated
metal ion
_____
d-orbitals
dz2 dx2- y2
only high spin
d-orbital energy level
diagram square planar
__
__
dx2- y2
dxy
__
E
__
isolated
metal ion
dz2
__
dxz dyz
_____
d-orbitals
only low spin
Crystal-Field Theory
square planar
Examples: Pd2+, Pt2+, Ir+,
and Au3+.
20_459
Tetrahedral Complexes
–
–
–
(a)
–
dz 2
–
– –– –
–
dxy
(b)
dx2 – y2
dxz
dyz
High spin
Low spin
• Spectrochemical Series: An order of ligand
field strength based on experiment:
Weak Field I-  Br- S2- SCN- Cl- NO3-
F-  C2O42- H2O NCS- CH3CN
NH3 en  bipy phen NO2- PPh3
CN- CO
Strong Field
H2 N
NH2
N
N
N
N
Ethylenediamine (en)
2,2'-bipyridine (bipy)
1.10 - penanthroline (phen)
Colors of Transition Metal Complexes
• Compounds/complexes that have color:
• absorb specific wavelengths of
visible light (400 –700 nm)
• wavelengths not absorbed are
transmitted and appear as color
Color and Magnetism
Color
Color of a complex depends on; (i) the metal, (ii) its
oxidation state & (iii) ligands (i.e., everything)
For example, pale blue [Cu(H2O)6]2+
versus dark blue [Cu(NH3)6]2+.
Partially filled d orbitals usually give rise to colored
complexes because they can absorb light from the
visible region of the spectrum.
Color and Magnetism
Color
Visible Spectrum
wavelength, nm
(Each wavelength corresponds to a different color)
400 nm
700 nm
higher energy
lower energy
White = all the colors (wavelengths)
Complexes and Color
The larger the gap, the shorter the wavelength of
light absorbed by electrons jumping from a lowerenergy orbital to a higher one.
[Ti(H2O)6]3+
Absorbs in green yellow.
Looks purple.
the spectrum for [Ti(H2O)6]3+ has a maximum
absorption at 510 nm
Absorbs green & yellow,
transmits all other wavelengths, the
complex is purple.
Crystal-Field Theory
[Ti(H2O)6]3+
Electronic Configurations of Transition
Metal Complexes
• d orbital occupancy depends on  and pairing
energy, P
– e-’s assume the electron configuration with the
lowest possible energy cost
– If  > P ( large; strong field ligand)
• e-’s pair up in lower energy d subshell first
– If  < P ( small; weak field ligand)
• e-’s spread out among all d orbitals before any pair up
d-orbital energy level diagrams
octahedral complex
1
d
d-orbital energy level diagrams
octahedral complex
2
d
d-orbital energy level diagrams
octahedral complex
3
d
d-orbital energy level diagrams
octahedral complex
4
d
high spin
<P
low spin
>P
d-orbital energy level diagrams
octahedral complex
5
d
high spin
<P
low spin
>P
d-orbital energy level diagrams
octahedral complex
6
d
high spin
<P
low spin
>P
d-orbital energy level diagrams
octahedral complex
7
d
high spin
<P
low spin
>P
d-orbital energy level diagrams
octahedral complex
8
d
d-orbital energy level diagrams
octahedral complex
9
d
d-orbital energy level diagrams
octahedral complex
10
d
20_441
Isomers
(same formula but different properties)
Structural
isomers
(different bonds)
Coordination
isomerism
Linkage
isomerism
Stereoisomers
(same bonds, different
spatial arrangements)
Geometric
(cis-trans)
isomerism
Optical
isomerism
Coordination complexes: isomers
Isomers: same atomic composition, different structures
Different composition!
We’ll check out the following
types of isomers:
Hydrate
Linkage
Cis-trans
Optical (Enantiomers)
Hydrate isomers:
Water in outer sphere (water
that is part of solvent)
Water in the inner
sphere water (water
is a ligand in the
coordination sphere
of the metal)
Structural Isomerism 1
• Coordination isomerism:
•
Composition of the complex ion varies.
•
[Cr(NH3)5SO4]Br
•
and [Cr(NH3)5Br]SO4
Coordination-Sphere Isomers
• Example
[Co(NH3)5Cl]Br vs. [Co(NH3)5Br]Cl
• Consider ionization in water
[Co(NH3)5Cl]Br  [Co(NH3)5Cl]+ + Br[Co(NH3)5Br]Cl  [Co(NH3)5Br]+ + Cl-
Structural Isomerism 2
• Ligand isomerism:
•
Same complex ion structure but point of
attachment of at least one of the ligands
differs.
•
•
[Co(NH3)4(NO2)Cl]Cl
and [Co(NH3)4(ONO)Cl]Cl
Linkage Isomers
Linkage isomers
Example:
S
C
N Bonding to metal may occur at
the S or the N atom
Bonding occurs from
N atom to metal
Bonding occurs from
S atom to metal
Linkage Isomers
[Co(NH3)5(NO2)]Cl2
[Co(NH3)5(ONO)]Cl2
Pentaamminenitrocobalt(III)
chloride
Pentaamminenitritocobalt(III)
chloride
Stereoisomers
• Stereoisomers
– Isomers that have the same bonds, but different
spatial arrangements
• Geometric isomers
– Differ in the spatial arrangements of the ligands
Stereoisomerism 1
• Geometric isomerism (cis-trans):
• Atoms or groups arranged differently spatially
relative to metal ion
• Pt(NH3)2Cl2
20_444
Cl
Cl
H3N
H3N
NH3
Co
Co
H3N
NH3
NH3
H3N
Cl
NH3
Cl
Cl
Cl
Co
Co
Cl
Cl
(a)
(b)
Geometric Isomers
cis isomer
trans isomer
Pt(NH3)2Cl2
Geometric Isomers
cis isomer
trans isomer
[Co(H2O)4Cl2]+
Stereoisomers: geometric isomers (cis and trans)
Cl
Cl
H3N Co NH3
H3N
NH3
Cl
H3N Co Cl
H3N
NH3
NH3
Cl-
Cl-
Stereoisomers
• Optical isomers
– isomers that are nonsuperimposable mirror
images
• said to be “chiral” (handed)
• referred to as enantiomers
– A substance is “chiral” if it does not have a “plane
of symmetry”
Stereoisomerism 2
• Optical isomerism:
•
Have opposite effects on plane-polarized
light
•
(no superimposable mirror images)
20_448
Mirror image
of right hand
Left hand
Right hand
Two coordination complexes which are enantiomers
NH3
H3N Co Cl
H2 O
Cl
H2O
NH3
Cl Co NH3
Cl
H2O
H2O
Chirality: the absence of a plane of symmetry
Enantiomers are possible
A molecule possessing a plane of symmetry is achiral and
a superimposible on its mirror image
Enantiomers are NOT possible
Are the following chiral or achiral structures?
NH3
Cl Co H2O
Cl
H2O
NH3
NH3
H3N Co Cl
H2 O
Cl
H2O
Plane of symmetry
Achiral (one structure)
NH3
Cl Co NH3
Cl
H2O
H2O
No plane of symmetry
Chiral (two enantiomer)
Enantiomers: non superimposable mirror images
A structure is termed chiral if it is not superimposable on
its mirror image
Structure
Mirror image
Of structure
Two chiral structures: non superimposable mirror images:
Enantiomers!
Which are enantiomers (non-superimposable mirror images)
and which are identical (superimposable mirror images)?
Mirror images
[Co(en)3]
5
1
5
3
3
2
4
1
6
4
2
6
Enantiomers: non superimposable mirror images
A structure is termed chiral if it is not superimposable on
its mirror image
Structure
Mirror image
Of structure
Two chiral structures: non superimposable mirror images:
Enantiomers!
20_449
N
N
N
Co
N
N
N
N
Mirror image
of Isomer I
N
N
N
N
Co
N
Co
N
N
Isomer I
N
N
N
Isomer II
N
Enantiomers
A molecule or ion that exists as a pair of enantiomers
is said to be chiral.
Cl
N
N
Co
N
N
N
N
Co
N
Co
N
cis
N
N
N
Isomer II cannot be
superimposed exactly
on isomer I. They are
not identical structures.
Cl
Cl
Cl
N
Cl
(a)
Cl
N
Cl
trans
The trans isomer and
its mirror image are
identical. They are not
isomers of each other.
Co
Cl
N
Cl
N
N
N
N
Co
Cl
Isomer I N
Isomer II N
(b)
Isomer II has the same
structure as the mirror
image of isomer I.