Transcript Document

Coordination Chemistry:
Isomerism and Structure
Chapter 7 and 19
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1. Isomerism
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A. Constitutional Isomers
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
SCNHard metal ions bind to the N
Soft metal ions bind to the S
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A. Constitutional Isomers
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
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A. Constitutional Isomers
IV. Coordination Isomers
Same metal
Formulation1Pt2+ : 2NH3 : 2 Cl[Pt(NH3)2Cl2]
[Pt(NH3)3Cl][Pt(NH3)Cl3]
[Pt(NH3)4][PtCl4]
Same metal but different
oxidation states
Formulation1Pt2+ : 1Pt4+ : 4NH3 : 6 Cl[Pt(NH3)4][PtCl6]
+2
+4
Different Metals
Formulation1Co3+ : 1Cr3+ : 6NH3 : 6 CN[Co(NH3)6][Cr(CN)6]
[Co(CN)6][Cr(NH3)6]
[Pt(NH3)4Cl2][PtCl4]
+4
+2
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B. Stereoisomers
I.
Enantiomers
 Optical isomers (chiral)

Non-superimposable mirror image
Square planar complex
If it were tetrahedral, it would not be chiral.
 Recall from group theory, something is chiral if
Has no improper rotation axis (Sn)
 Has no mirror plane (S1)
 Has no inversion center (S2)
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B. Stereoisomers
II. Diastereomers
a. Geometric isomers
 4-coordinate complexes
 Cis and trans isomers of square-planar complexes (cis/transplatin)
cis
(anticancer agent)
trans
 Chelate rings can enforce a cis structure if the chelating ligand is too small to span the
trans positions
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B. Stereoisomers
II. Diastereomers
a. Geometric isomers
 6-coordinate complexes
Facial(fac) arrangement of ligands
Two sets of ligands segregated to two
different faces.
Meridional(mer) arrangement of ligands
Two sets of ligands segregated into two
perpendicular planes.
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B. Stereoisomers
II. Diastereomers
a. Geometric isomers
 6-coordinate complexes
 Different arrangements of chelating ring
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B. Stereoisomers
III. Conformational isomers
 Because many chelate rings are not planar, they can have different conformations in
different molecules, even in otherwise identical molecules.
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B. Stereoisomers
Conformational isomers
 Ligands as propellers
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B. Stereoisomers
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
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C. Separation of Isomers
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
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D. Identification of Isomers
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.
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3. Coordination Numbers and Structures
I.
Common Structures
Factors involved:
 VSEPR fails for transition metal complexes
 Occupancy of metal d orbitals
dx2-y2
dxz
dz 2
dyz
dxy
 Sterics
 Crystal packing effects
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3. Coordination Numbers and Structures
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.
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3. Coordination Numbers and Structures
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
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3. Coordination Numbers and Structures
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 sixcoordination
• Cis and trans isomers
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3. Coordination Numbers and Structures
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:
Sq Pyr Geometry favored by:
d1, d2, d3, d4, d8, d9, d10 metal ions
d6 (low spin) metal ions
Electronegative ligands prefer axial position
Big ligands prefer equatorial position
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3. Coordination Numbers and Structures
c. Coordination Number = 6
i. 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
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3. Coordination Numbers and Structures

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)
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3. Coordination Numbers and Structures
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
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3. Coordination Numbers and Structures
c. Coordination Number = 8
Not common
i.
Cube
 CsCl
ii. Trigonal dodecahedron
iii. Square antiprism
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3. Coordination Numbers and Structures
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
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3. Coordination Numbers and Structures
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
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4. Bioinorganic Chemistry
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.
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4. Bioinorganic Chemistry
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.
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