Coordination Chemistry:Isomerism and Structure

Coordination Chemistry:Isomerism and Structure

Chapter 7 and 19

Chapter 7 and 19

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1. Isomerism

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A. Constitutional Isomers

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I. Linkage (Ambidentate) Isomers

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

SCN-

Hard metal ions bind to the N

Soft metal ions bind to the S

A. Constitutional Isomers

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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

A. Constitutional Isomers

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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]

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

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B. Stereoisomers

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I. Enantiomers Optical isomers (chiral)

Non-superimposable mirror image

Recall from group theory, something is chiral if Has 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

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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

cis(anticancer agent)

trans

B. Stereoisomers

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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

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II. Diastereomers

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

B. Stereoisomers

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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

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Conformational isomers Ligands as propellers

B. Stereoisomers

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Conformational isomers Ligand symmetry can be changed by coordination. Coordination may make

ligands chiral as exhibited by the four-coordinate nitrogens.

Conformational isomers Conformational isomers

Geometric isomers

C. Separation of Isomers

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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

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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

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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

3. Coordination Numbers and Structures

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

i. Coordination number = 1• Rare for complexes in condensed phases (solids and liquids).• Often solvents will try to coordinate.

3. Coordination Numbers and Structures

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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

3. Coordination Numbers and Structures

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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 configuration Ones 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

3. Coordination Numbers and Structures

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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

3. Coordination Numbers and Structures

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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

3. Coordination Numbers and Structures

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Trigonal distortions (Elongation or compression along C3 axis)

• Trigonal prism (D3h) Favored by chelates with small bite angles or specific types of ligands

• Trigonal antiprism (D3d)

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

3. Coordination Numbers and Structures

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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

3. Coordination Numbers and Structures

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c. Coordination Number = 8 Not common

i. Cube CsCl

ii. Trigonal dodecahedron

iii. Square antiprism

3. Coordination Numbers and Structures

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II. Rules of thumb

Factors favoring low coordination numbers:

a. Soft ligands and soft metals (low oxidation states)b. Large bulky ligandsc. Counterions of low basicity

“Least coordinating anion”

BArF

3. Coordination Numbers and Structures

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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

4. Bioinorganic Chemistry

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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

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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.