lecture-october 22- isomerism and coordination number€¦ · optical rotatory dispersion (ord):...
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Chapter 19-Coordination Chemistry:
Isomerism and Coordination Number
Chapter 19-Coordination Chemistry:
Isomerism and Coordination Number1
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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
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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)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
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]
[Cr(NH3)6][Co(CN)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 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
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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 (constrained bite angle)
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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Coordination may make ligands chiral as exhibited by the four-coordinate nitrogens.
Optical isomers Optical isomers
Geometric isomers
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 Ligands as propellers
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).• In solution, often solvents will try to coordinate. • Bulky ligands can play a big role here.
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 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
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 smallbite angles or specific types ofligands
• 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 ligands
c. 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 to not compete with the metal for ligand interaction
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.