Date post: | 23-Dec-2015 |
Category: |
Documents |
Upload: | james-nash |
View: | 231 times |
Download: | 0 times |
Crystal Field Theory• Focus: 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
_ _ _
_ _
dyzdxzdxy
dz2 dx2- y2
_ _ _ _ _
isolated metal ion
d-orbitals
E
d-orbital energy level diagram for tetrahedral
only high spin
dyzdxz
dxy
dz2
dx2- y2
_ _ _ _ _
isolated metal ion
d-orbitals
E
d-orbital energy level diagram square planar
__
__
__
____
only low spin
Crystal-Field Theory
square planar
Examples: Pd2+, Pt2+, Ir+, and Au3+.
20_459
–
–
–
––
–– ––
–
dz2 dx2 – y2
dxy dyzdxz
(a) (b)
Tetrahedral Complexes
High spin Low spin
• Spectrochemical Series: An order of ligand field strength based on experiment:
I- Br- S2- SCN- Cl- NO3- F-
C2O42- H2O NCS- CH3CN NH3 en
bipy phen NO2- PPh3 CN- CO
Weak Field
Strong Field
N N
2,2'-bipyridine (bipy)
NH2 NH2
Ethylenediamine (en)
N
N
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 ColorColor 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
White = all the colors (wavelengths)
400 nm 700 nm
wavelength, nm
higher energy lower energy
(Each wavelength corresponds to a different color)
Complexes and ColorThe larger the gap, the shorter the wavelength of light absorbed by electrons jumping from a lower-energy 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 diagramsoctahedral complex
d1
d-orbital energy level diagramsoctahedral complex
d2
d-orbital energy level diagramsoctahedral complex
d3
d-orbital energy level diagramsoctahedral complex
d4
high spin < P
low spin
> P
d-orbital energy level diagramsoctahedral complex
d5
high spin < P
low spin
> P
d-orbital energy level diagramsoctahedral complex
d6
high spin < P
low spin
> P
d-orbital energy level diagramsoctahedral complex
d7
high spin < P
low spin
> P
d-orbital energy level diagramsoctahedral complex
d8
d-orbital energy level diagramsoctahedral complex
d9
d-orbital energy level diagramsoctahedral complex
d10
20_441
Isomers(same formula but different properties)
Stereoisomers(same bonds, differentspatial arrangements)
Structuralisomers
(different bonds)
Opticalisomerism
Geometric(cis-trans)isomerism
Linkageisomerism
Coordinationisomerism
Coordination complexes: isomers
Isomers: same atomic composition, different structures
We’ll check out the following types of isomers:HydrateLinkageCis-transOptical (Enantiomers)
Different composition!
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)
Hydrate isomers:
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
Bonding to metal may occur at the S or the N atom
Example: C NS
Bonding occurs from N atom to metal
Bonding occurs from S atom to metal
Linkage Isomers
[Co(NH3)5(NO2)]Cl2
Pentaamminenitrocobalt(III)chloride
[Co(NH3)5(ONO)]Cl2
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
H3N
Co
H3N
NH3
NH3
Cl
Cl
H3N
Co
H3N
NH3
Cl
Cl
NH3
Cl
Cl
Co
Cl
Cl
Co
(a) (b)
cis isomer trans isomerPt(NH3)2Cl2
Geometric Isomers
cis isomer trans isomer[Co(H2O)4Cl2]+
Geometric Isomers
Stereoisomers: geometric isomers (cis and trans)
Cl-
Cl
Co Cl
NH3H3N
H3N
NH3
Cl
Co NH3NH3H3N
H3N
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
Left hand Right hand
Mirror imageof right hand
Two coordination complexes which are enantiomers
NH3
Co Cl
ClH2O
H3N
H2O
NH3
Co NH3H2OCl
Cl
H2O
Plane of symmetry Achiral (one structure)
Chirality: the absence of a plane of symmetryEnantiomers are possible
A molecule possessing a plane of symmetry is achiral and a superimposible on its mirror image
Enantiomers are NOT possible
No plane of symmetryChiral (two enantiomer)
NH3
Co H2O
H2OCl
Cl
NH3
NH3
Co Cl
ClH2O
H3N
H2O
NH3
Co NH3H2OCl
Cl
H2O
Are the following chiral or achiral structures?
Enantiomers: non superimposable mirror images
A structure is termed chiral if it is not superimposable on its mirror image
Two chiral structures: non superimposable mirror images:
Enantiomers!
Structure Mirror imageOf structure
Which are enantiomers (non-superimposable mirror images) and which are identical (superimposable mirror
images)?
Mirror images [Co(en)3]
1
2
1
23
3
4
4
5
5
6
6
Enantiomers: non superimposable mirror images
A structure is termed chiral if it is not superimposable on its mirror image
Two chiral structures: non superimposable mirror images:
Enantiomers!
Structure Mirror imageOf structure
20_449
N
N
N
N
N
NCo
N
N
N
N
N
NCo
Mirror imageof Isomer I
Isomer I Isomer II
N
N
N
N
N
NCo
Enantiomers
A molecule or ion that exists as a pair of enantiomers is said to be chiral.
20_450
Cl
Cl
N
N
N
NCo
Cl
Cl
N
N
N
NCo
Cl
Cl
N
N
N
NCo
Cl
Cl
N
N
N
NCo
Cl
Cl
N
N
N
NCo
Isomer IIIsomer I
cistrans
Isomer II cannot besuperimposed exactlyon isomer I. They arenot identical structures.
The trans isomer andits mirror image areidentical. They are notisomers of each other.
Isomer II has the samestructure as the mirrorimage of isomer I.(b)(a)