Download Document

Coordination Chemistry:
Isomerism and Structure
Chapter 7 and 19
1
1. Isomerism
2
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
3
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
4
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
5
6
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)
7
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
8
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.
9
B. Stereoisomers
II. Diastereomers
a. Geometric isomers
 6-coordinate complexes
 Different arrangements of chelating ring
10
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.
11
B. Stereoisomers
Conformational isomers
 Ligands as propellers
12
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
13
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
14
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.
15
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
16
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.
17
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
18
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
19
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
20
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
21
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)
22
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
23
3. Coordination Numbers and Structures
c. Coordination Number = 8
Not common
i.
Cube
 CsCl
ii. Trigonal dodecahedron
iii. Square antiprism
24
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
25
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
26
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.
27
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.
28