Transcript Stereoisomers

• The importance of transition metal complexes in biochemistry
is an extremely active area of research.
1.
Catalytic cofactors: Many reactions require trace elements as catalytic cofactors. For
example:
a)
b)
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3.
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Zinc is required in over 200 reactions including synthesis of proteins, taste perception,
prostrate reproductive health, metabolizing alcohol, and protecting against copper and heavy
metal toxicity such as cadmium and lead. Zinc occurs in greater amounts than any other
trace mineral except iron.
Manganese is required to synthesize connective tissue and bones (collagen).
Oxidation/reduction: Some metal ions, particularly iron, copper, and manganese are
involved in the energy metabolism of cells. Iron is involved in the electron
transport that ultimately converts oxygen to water. Copper participates in electron transport
as
well as synthesis of nerve membranes and formation of collagen.
Oxygen binding and transport: Oxygen is carried by the red cells of the blood from the
lungs bound to hemoglobin which contains iron at its active heme center. Oxygen
is
released to the tissues where it is picked up by a similar protein, myoglobin,
before it accepts electrons and protons to form water.
Metabolic regulation: Iron, copper and zinc can regulate the activities of protein and
nucleic acid synthesis. Proper immune response requires these trace elements.
Structural integrity: The three dimensional architecture of proteins and nucleic acids
depends upon zinc and manganese as well as iron and copper. These metals bind
and
hold large molecules in active configurations. An example is the requirement of
zinc for proper conformation of the taste- bud proteins in the tongue. Without zinc, taste
and smell are lost. Iodide is incorporated into the amino acids which synthesize thyroid
hormones.
Important Terms
• A ligand is a Lewis base that coordinates to a central metal atom or ion.
• A donor atom is the atom in a ligand that donate a lone pair of electrons
to form a coordinate covalent bond.
• A unidentate ligand is a ligand that can bind through only one atom.
• A polydentate ligand is a ligand that can bind through more than one
donor atom.
• There are known examples of bidentate, tridentate, quadridentate,
quinquedentate, and sexidentate ligands.
• Chelate complexes are complexes that have a metal atom or ion and
polydentate ligand(s) that form rings.
• The coordination number is the number of donor atoms coordinated to
a metal atom or ion.
• A coordination sphere includes the metal atom or ion and the ligands
coordinated to it. The coordination sphere does not include
uncoordinated counter ions.
Important Terms
• For the complex compound K3[Co(CN)6] the coordination
number is _________, and the coordination sphere is _______.
Nomenclature
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Rules for Naming Complex Species
Cations (+ ions) are named before anions (- ions).
Coordinated ligands are named in alphabetical order.
–
Prefixes that specify the number of each kind of ligand (di = 2, tri = 3, tetra = 4, penta = 5, hexa =
6, etc.) are not used in alphabetizing
–
Prefixes that are part of the name of the ligand, such as in diethylamine, are used to alphabetize
the ligands.
For complicated ligands, especially those that have a prefix such as di or tri as part of the ligand
name, these prefixes are used to specify the number of those ligands that are attached to the central
atom.
–
bis = 2 tris = 3 tetrakis = 4 pentakis = 5 hexakis = 6
The names of most anionic ligands end in the suffix -o.
–
Examples of ligands ending in –o are:
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Cl- chloro
S2- sulfido
O2- oxo
The names of most neutral ligands are unchanged when used in naming the complex.
–
There are several important exceptions to this rule including:
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NH3 ammine
H2O aqua
The oxidation number of a metal that exhibits variable oxidation states is designated by a Roman
numeral in parentheses following the name of the complex ion or molecule.
If a complex is an anion, the suffix "ate" ends the name.
 No suffix is used in the case of a neutral or cationic complex.
 Usually, the English stem is used for a metal, but if this would make the name awkward, the Latin
stem is substituted. ferrate instead of ironate plumbate instead of leadate
Important Terms
Typical Simple Ligands
Ion/Molecule
Name
Name as a Ligand
NH3
ammonia
ammine
CO
carbon monoxide
carbonyl
Cl-
chloride
Chloro
CN-
cyanide
cyano
F-
fluoride
fluoro
OH-
hydroxide
hydroxo
NO
nitrogen monoxide
nitrosyl
NO2-
nitrite
nitro
PH3
phosphine
phosphine
Nomenclature
• Name the following compounds:
Na3[Fe(Cl)6]
[Ni(NH3)4(OH2)2](NO3)2
Nomenclature
• Write formulas for the following compounds:
potassium hexacyanochromate(III)
tris(ethylenediammine) cobalt(III) nitrate
Structures
• The structures of coordination compounds are controlled
primarily by the coordination number of the metal.
• Usually the structures can be predicted by VSEPR theory
(Chapter 8).
– The geometries and hybridizations for common
coordination numbers are summarized in this table.
CN
Geometry
Hybridization
Example
2
Linear
sp
[Ag(NH3)2]+
4
Tetrahedral
sp3
[Cd(NH3)4]2+
4
square planar
sp2d
[Cu(OH2)4]2+
5
trigonal bipyramid
sp3d
Fe(CO)5
5
Square pyramidal
sp2d2
[Mn(Cl)5]3-
6
Octahedral
sp3d2
[Fe(CN)6]4-
Structures
• Sketch the shape of the hexacyanaochromate(III) ion.
Isomerism in Coordination Compounds
•
Isomers . two or more forms of a compound having the same
composition
• Structural isomers involve different atom to ligand bonding sequences.
–
hydration isomers isomers
»
–
exchange ion between ligand and anion
coordination isomers
»
–
coordination sphere
isomers
ionization isomer
»
–
exchange water as ligand
and hydrate
denote an exchange of ligands between the coordination spheres of the
cation and anion.
linkage isomers
»
different ligands or different
attachment of ligands
• Stereoisomers (identical bonding)
• geometrical isomers
• optical isomers
Structural (Constitutional) Isomers
• Hydrate isomers are a special case of ionization isomers in which
water molecules may be changed from inside to outside the
coordination sphere.
• For example:
– [Cr(OH2)6]Cl3 vs.
– [Cr(OH2)5Cl]Cl2. H2O vs.
– [Cr(OH2)4Cl2]Cl2. 2H2O
• Note whether the water molecule(s) are inside or outside the
coordination sphere.
Structural (Constitutional) Isomers
[Cr(OH2)6]Cl3
[Cr(OH2)5Cl]Cl2. H2O
[Cr(OH2)5Cl]Cl2. H2O
[Cr(OH2)4Cl2]Cl2. 2H2O
Structural (Constitutional) Isomers
Ionization or Ion-Ion Exchange Isomers
[Pt(NH3)4Cl2]Br2 compared to [Pt(NH3)4Br2]Cl2
Note where the Cl’s and Br’s are in the structures, that is what makes these
two species isomers.
• [Pt(NH3)4Cl2]Br2
[Pt(NH3)4Br2]Cl2
Structural (Constitutional) Isomers
• Coordination isomers denote an exchange of ligands between
the coordination spheres of the cation and anion.
• For example look at these two isomers:
[Pt(NH3)4][PtCl6] vs [Pt(NH3)4Cl2][PtCl4]
• The isomeric distinction is whether the ligands are on the
cation or the anion.
Structural (Constitutional) Isomers
[Pt(NH3)4][PtCl6]
[Pt(NH3)4Cl2][PtCl4]
Structural (Constitutional) Isomers
• Linkage isomerism if a ligand contains more than one atom with a free
electron pair, the ligand may be bound to the central atom via the different atoms.
N
bonding via N
bonding via O
O
O
C
S C
N
N
nitronitrito-
bonding via C
bonding via N
bonding via S
bonding via N
cyanoisocyanothiocyanatoisothiocyanato-
Structural (Constitutional) Isomers
[Co(NH3)5ONO]Cl2
[Co(NH3)5NO2]Cl2
Stereoisomers
• Stereoisomers are isomers that have different spatial
arrangements of the atoms relative to the central atom.
• Complexes with only simple ligands can occur as
stereoisomers only if they have coordination numbers equal to
or greater than four.
• Geometrical or positional isomers are stereoisomers that are
not optical isomers.
• Cis-trans isomers have the same kind of ligand either adjacent
to each other (cis) or on the opposite side of the central metal
atom from each other (trans).
• Note where the ligands are positioned relative to the central
atom.
Stereoisomers
cis- [Pt(NH3)2Cl2]
trans-[Pt(NH3)2Cl2]
Stereoisomers
• Other types of isomerism can occur in octahedral complexes.
• Complexes of the type [MA2B2C2] can occur in several
geometric isomeric forms:
– trans- trans- trans– cis- cis- cis– cis- cis- trans-
Stereoisomers
trans-diammine-trans-diaqua-trans-dichlorocobalt(III) ion
cis-diammine-cis-diaqua-cis-dichlorocobalt(III) ion
Stereoisomers
trans-diammine-cis-diaqua-cis-dichlorocobalt(III) ion
cis-diammine-cis-diaqua-trans-dichlorocobalt(III) ions
Stereoisomers
cis-diammine-trans-diaqua-cis-dichlorocobalt(III) ions
• Octahedral complexes can exhibit another type of geometric isomerism
- mer-fac isomerism.
•mer isomerism involves all three similar ligand lying in the same plane, or
meridianl like a globe.
•fac facial involves a grouping of three similar ligands that are arranged on a
triangular face of the octrahedron
fac and mer-Co(NH3)3Cl3
Stereoisomers
• Optical isomers are mirror images of each other that are not
superimposable.
• The cis-diammine-cis-diaqua-cis-dichlorocobalt(III) ion has two
different forms called optical isomers or enantiomers.
• Separate equimolar solutions of the two isomers rotate plane polarized light
by equal angles but in opposite directions.
– The phenomenon of rotation of polarized light is called optical
activity.
Stereoisomers
• These are the optical isomers of:
cis-diammine-cis-diaqua-cis-dichlorocobalt(III) ion