coordination chemistry: nomenclature, isomerism, and structure
DESCRIPTION
Coordination Chemistry: Nomenclature, Isomerism, and Structure. Chapter 9. 1. Nomenclature Rules. For charged molecules, the cation comes first followed by the anion. The following rules apply to both neutral and charged molecules: - PowerPoint PPT PresentationTRANSCRIPT
Coordination Chemistry:Nomenclature, Isomerism, and
Structure
Chapter 9
1
1. Nomenclature Rules
2
A. For charged molecules, the cation comes first followed by the anion.
The following rules apply to both neutral and charged molecules:
B. The elemental formulation has the inner coordination sphere in brackets. [Pt(NH3)4]Cl2
When writing the name, the ligands within the coordination sphere are written before the metal and are listed in alphabetical order.
tetraammineplatinum(II) chloride
C. Ligand names (refer to handout). Monodentate: Ligands with one point of attachment Chelates (Bidentate…multidentate): Ligands with two or more points of
attachment
1. Nomenclature Rules
3
C. The number of ligands of each kind is indicated by prefixes using the following table. A B
Use prefixes in column A for simple cases.
Use prefixes in column B for ligands with names that already use prefixes from column A.
[Co(en)2Cl2]+
Dichlorobis(ethylenediamine)cobalt(III)
Always use prefixes in column B when the name of a ligand begins with a vowel.
[Rh(en)3]3+
Tris(ethylenediamine)rhodium(III)
1. Nomenclature Rules
4
D. Ligands are written in alphabetical order-according to the ligand name, not the prefix.
E. Anionic ligands are given an o suffix.
Neutral ligands retain their usual name
Coordinated water is called aqua
Coordinated ammonia is called ammine
1. Nomenclature Rules
5
F. Designate the metal oxidation state after the metal.
[PtClBr(NH3)(H2O)] Ammineaquabromochloroplatinum(II)
[Pt(NH3)4]2+
Tetraammineplatinum(II)
If the molecule is negatively charged, the suffix –ate is added to the name
[Pt(NH3)Cl3]-
Amminetrichloroplatinate(II)
1. Nomenclature Rules
6
Special names for metals when in a negatively charged molecule:
Copper (Cu): Cuprate
Iron (Fe): ferrate
Silver (Ag): argentate
Lead (Pb): Plumbate
Tin(Sn): Stannate
Gold(Au): Aurate
1. Nomenclature Rules
7
G. Prefixes designate adjacent (cis-) and opposite (trans-) geometric locations
cis-diamminedichloroplatinum(II) is an anticancer agent. The trans isomer is not.
1. Nomenclature Rules
8
H. Bridging ligands between two metal ions have the prefix μ
μ-amido-μ-hydroxobis(tetraaminecobalt)(IV)
2. Isomerism
9
Ligand isomers
2. Isomerism
10
Conformational Isomers
Geometric Isomers
cis-trans
fac-mer
A. Constitutional Isomers
11
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
Solvent can influence the point of attachmentFor SCN- : M-S bonds favored in solvents of high dipole moment
M-N bonds favored in solvents of low dipole moment
A. Constitutional Isomers
12
II. Ligand Isomers
III. 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
IV. 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
NH2
NH2 NH2 NH2
VS.
A. Constitutional Isomers
13
V. Coordination Isomers
Same metalFormulation- Pt(NH3)2Cl2
[Pt(NH3)2Cl2]
[Pt(NH3)3Cl][Pt(NH3)Cl3]
[Pt(NH3)4][PtCl4]
Same metal but different oxidation states
[Pt(NH3)4][PtCl6]+2 +4
[Pt(NH3)4Cl2][PtCl4]+4 +2
Different Metals
[Co(NH3)6][Cr(CN)6]
[Co(CN)6][Cr(NH3)6]
B. Stereoisomers
14
I. Enantiomers Optical isomers (chiral)
Non-superimposable mirror image
Recall from group theory, something is chiral if a. Has no improper rotation axis (Sn) b. Has no mirror plane (S1) c. Has no inversion center (S2)
Square planar complex
If it were tetrahedral, it would not be chiral.
B. Stereoisomers
15
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
B. Stereoisomers
16
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
17
II. Diastereomers
a. Geometric isomers 6-coordinate complexes Different arrangements of chelating ring
B. Stereoisomers
18
b. 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
19
b. Conformational isomers Ligands as propellers
B. Stereoisomers
20
b. Conformational isomers Ligand symmetry can be changed by coordination. Coordination may make
ligands chiral as exhibited by the four-coordinate nitrogens.
C. Separation of Isomers
21
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
C. Identification of Isomers
22
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
23
I. Common Structures Factors involved:
VSEPR considerations
Occupancy of metal d orbitals
Sterics
Crystal packing effects
3. Coordination Numbers and Structures
24
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
25
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
26
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
27
c. Coordination Number = 5 Trigonal bipyramidal vs square pyramidal
• Can be highly fluxional• Isolated complexes tend to be a distorted form of one or the other
D3h C4vTBP 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
28
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
29
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
30
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
31
c. Coordination Number = 8 Not common
i. Cube CsCl
ii. Trigonal dodecahedron
iii. Square antiprism
3. Coordination Numbers and Structures
32
II. Rules of thumb
Factors favoring low coordination numbers:
a. Soft ligands and metals in low oxidation statesb. Large bulky ligandsc. Counterions of low basicity
“Least coordinating anion”
BArF
3. Coordination Numbers and Structures
33
II. Rules of thumb
Factors favoring high coordination numbers:
a. Hard ligands and metals in high oxidation states
b. Small ligands
c. Large nonacidic cations