lecture-october 22- isomerism and coordination number€¦ · optical rotatory dispersion (ord):...

$: Lecture-October 22- Isomerism and Coordination Number€¦ · Optical rotatory dispersion (ORD): Caused by a difference in the refractive indices of the right and left circularly$
Chapter 19-Coordination Chemistry:

Isomerism and Coordination Number

Chapter 19-Coordination Chemistry:

Isomerism and Coordination Number1

2

Review of the Previous Lecture

1. Nomenclature

2. Thermodynamics of metal ligand interactions

Distinguished between affinity constants for one step metal-ligand binding versus cumulative steps

Explored factors that drive metal ligand complex formation

HSAB Theory

Chelate Effect

Introduced how ligand binding can stabilize metal oxidation states

1. Isomerism

3

A. Constitutional Isomers

4

I. Linkage (Ambidentate) Isomers

A ligand can bind in more than one way[Co(NH3)5(NO2)]2+

Co-NO2 Nitro isomer; yellow compound

Co-ONO Nitrito isomer; red compound

The binding at different atoms can be due to the Hard/Soft-ness of the metal ions

Example: SCN-

Hard metal ions bind to the N

Soft metal ions bind to the S


5

II. Ionization Isomers Difference in which ion is included as a ligand and which is present to balance the overall

charge

[Co(NH3)5Br]SO4 vs [Co(NH3)5SO4]Br

III. Solvate (Hydrate) Isomers The solvent can play the role of ligand or as an additional crystal occupant

[CrCl(H2O)5]Cl2· H2O vs [Cr(H2O)6]Cl3


6

IV. Coordination Isomers

Same metal

Formulation-1Pt2+ : 2NH3 : 2 Cl-

[Pt(NH3)2Cl2]

[Pt(NH3)3Cl][Pt(NH3)Cl3]

[Pt(NH3)4][PtCl4]

Same metal but different oxidation states

Formulation-1Pt2+ : 1Pt4+ : 4NH3 : 6 Cl-

[Pt(NH3)4][PtCl6]+2 +4

[Pt(NH3)4Cl2][PtCl4]+4 +2

Different Metals

Formulation-1Co3+ : 1Cr3+ : 6NH3 : 6 CN-

[Co(NH3)6][Cr(CN)6]

[Cr(NH3)6][Co(CN)6]

7

B. Stereoisomers

8

I. Enantiomers Optical isomers (chiral)

Non-superimposable mirror image

Recall from group theory, something is chiral ifHas no improper rotation axis (Sn) Has no mirror plane (S1) Has no inversion center (S2)

Square planar complex

If it were tetrahedral, it would not be chiral.

B. Stereoisomers

9

II. Diastereomers

a. Geometric isomers 4-coordinate complexes

Cis and trans isomers of square-planar complexes (cis/transplatin)

Chelate rings can enforce a cis structure if the chelating ligand is too small to span the trans positions (constrained bite angle)

cis(anticancer agent)

trans

B. Stereoisomers

10

II. Diastereomers

a. Geometric isomers 6-coordinate complexes

Facial(fac) arrangement of ligands Meridional(mer) arrangement of ligands

Two sets of ligands segregated to two different faces.

Two sets of ligands segregated into two perpendicular planes.

B. Stereoisomers

11

II. Diastereomers

a. Geometric isomers 6-coordinate complexes Different arrangements of chelating ring

B. Stereoisomers

12

Coordination may make ligands chiral as exhibited by the four-coordinate nitrogens.

Optical isomers Optical isomers

Geometric isomers

B. Stereoisomers

13

III. Conformational isomers Because many chelate rings are not planar, they can have different conformations in

different molecules, even in otherwise identical molecules.

M

HN

NH

H2C

H2C

B. Stereoisomers

14

Conformational isomers Ligands as propellers

B. Stereoisomers

15

Conformational isomers Ligands as propellers

C. Separation of Isomers

16

I. Fractional crystallization can separate geometric isomers.

a. Strategy assumes isomers have different solubilities in a specific solvent mixture and will not co-crystallize.

b. Ionic compounds are least soluble when the positive and negative ions have the same size and magnitude of charge. Large cations will crystallize best with large anions of the same charge.

II. Chiral isomers can be separated using

a. Chiral counterions for crystallization

b. Chiral magnets

D. Identification of Isomers

17

I. X-ray crystallography

II. Spectroscopic methods

In general, crystals of different handedness rotate light differently.

a. Optical rotatory dispersion (ORD): Caused by a difference in the refractive indices of the right and left circularly polarized light resulting from plane-polarized light passing through a chiral substance.

b. Circular dichroism (CD): Caused by a difference in the absorption of right-and left-circularly polarized light.

3. Coordination Numbers and Structures

18

I. Common Structures

Factors involved:

VSEPR fails for transition metal complexes

Occupancy of metal d orbitals

Sterics

Crystal packing effects

dx2-y2 dxz dz2 dyz dxy


19

a. Low coordination numbers Making bonds makes things more stable.

i. Coordination number = 1• Rare for complexes in condensed phases (solids and liquids).• In solution, often solvents will try to coordinate. • Bulky ligands can play a big role here.


20

ii. Coordination number = 2• Also rare• Ag(NH3)2

+; d10 metal• Linear geometry

iii. Coordination number = 3• [Au(PPH3)3]+; d10 metal• Trigonal planar geometry


21

b. Coordination Number = 4 Avoid crowding large ligands around the metal

i. Tetrahedral geometry is quite common• Favored sterically• Favored for L = Cl-, Br-, I- and

M = noble gas or pseudo noble gas configurationOnes that don’t favor square planar geometry by ligand field stabilization energy

ii. Square planar• Ligands 90° apart• d8 metal ions; M(II)• Smaller ligands, strong field ligands that π-bond well to compensate for no six-

coordination• Cis and trans isomers


22

c. Coordination Number = 5 Trigonal bipyramidal vs square pyramidal

• Can be highly fluxional in that they interconvert • Isolated complexes tend to be a distorted form of one or the other

D3h C4v

TBP Geometry favored by:

d1, d2, d3, d4, d8, d9, d10 metal ions

Electronegative ligands prefer axial position

Big ligands prefer equatorial position

Sq Pyr Geometry favored by:

d6 (low spin) metal ions


23

c. Coordination Number = 6i. Mostly octahedral geometry (Oh)

Favored by relatively small metals Isomers

ii. Distortions from Oh Tetragonal distortions: Elongations or compressions along Z axis

• Symmetry becomes D4h


24

Trigonal distortions (Elongation or compression along C3 axis)

• Trigonal prism (D3h)Favored by chelates with smallbite angles or specific types ofligands

• Trigonal antiprism (D3d)

Rhombic distortions (Changes in two C4 axes so that no two are equal; D2h)


25

c. Coordination Number = 7 Not common

i. Pentagonal bipyramid

ii. Capped octahedron 7th ligand added @ triangular face

iii. Capped trigonal prism 7th ligand added @ rectangular face


26

c. Coordination Number = 8 Not common

i. Cube CsCl

ii. Trigonal dodecahedron

iii. Square antiprism


27

II. Rules of thumb

Factors favoring low coordination numbers:

a. Soft ligands and soft metals (low oxidation states)

b. Large bulky ligands

c. Counterions of low basicity “Least coordinating anion”

BArF


28

II. Rules of thumb

Factors favoring high coordination numbers:

a. Hard ligands and hard metals (high oxidation states)

b. Small ligands

c. Large nonacidic cations to not compete with the metal for ligand interaction

4. Bioinorganic Chemistry

29

Metal coordination in biology obeys coordination trends but expect distorted geometries.

Classical example is hemoglobin for oxygen transport:

2+

Intermediate metal ion bound by intermediate ligand; stabilized by the reducing environment of blood cells.

2+

4. Bioinorganic Chemistry

30

In hemoglobin, a coordination site is made available to bind and transport O2 . The metal oxidation state of 2+ is important for this binding process.

lecture-october 22- isomerism and coordination number€¦ · optical rotatory dispersion (ord):...

Documents