crystal field theory focus: energies of the d orbitals assumptions 1.ligands:negative point charges...

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)