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Transition Metal Complexes

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Transition Metal
Complexes I
The structures, nomenclature and isomers of
coordination compounds
Coordination Compounds
Any compound containing a metal atom or
ion with one or more ligands is called a
coordination compound or complex. The ligands
donate electrons to the metal via coordinate
covalent bonds.
Coordination Compounds
The structures of these compounds was not
always evident. Ions or molecules might be
directly bonded to the metal, or serve as a
counter ion for an ionic salt.
[Mn(OH2)6] SO4 sulfate ion is outer sphere
[Mn(OH2)5 SO4].H2O sulfate ion is inner sphere
Coordination Compounds
Early chemists approached transition metal
complexes using the concept of “valences”
adapted from main group metals. Metals with a
+3 charge, such as iron(III) or cobalt(III) were
believed to only make three bonds.
A compound such as [Co(NH3)6]Cl3 was
thought to have three Co-Cl bonds, with no way
to explain the bonding of ammonia in the
compound.
Coordination Compounds
One approach by Blomstrand proposed
chains of linked ammonia molecules, with the
nitrogens having five bonds and connecting a
chloride to the metal.
Alfred Werner proposed that the ammonia
molecules could bond strongly and directly to
the metal, with chlorides either directly bonded,
or loosely bonded and ionic in solution.
Coordination Compounds
Coordination Compounds
Jorgensen supported Blomstrand’s approach,
and Werner, in order to support his theory,
synthesized new compounds and studied their
isomers. Eventually, in 1907, Werner prevailed
and proved the octahedral geometry of
coordination compounds.
Coordination Compounds
Alfred Werner (1866-1919) also determined
the formulas and structures of many transition
metal compounds by studying their isomers.
Due to the existence of a variety of structural
isomers, he proposed that complexes must have
square planar and octahedral shapes.
Possible Structures for 6Coordinate Cobalt
Possible Structures for 6Coordinate Cobalt
Possible Structures for 6Coordinate Cobalt
Proposed Structures for 6Coordinate Cobalt
Structure of Compounds
Composition
# ions
Modern formulation
PtCl2.4NH3
3
[Pt(NH3)4]2+ 2Cl-
PtCl2.3NH3
2
[PtCl(NH3)3]+Cl-
PtCl2.2NH3
0
[PtCl2 (NH3)2]
(2 forms)
Nomenclature of Coordination
Compounds
There is a separate system for naming
coordination compounds:
prefix indicating + ligand name + metal + (oxidation #
# of ligands
in roman
numerals)
or
“
“ “
+“ “ “
+ “ “ “ + (charge of
complex)
Nomenclature of Coordination
Compounds
1. If ionic, the positive ion is named first, then the
negative ion.
2. The inner coordination sphere is indicated by
square brackets. In the formula, the metal is
written first, followed by the ligands. In naming,
the ligands are named first, then the metal.
Nomenclature of Coordination
Compounds
3. Prefixes: If the ligand itself contains a prefix in
its name (ex. dimethyl amine), then the prefix to
indicate the number of ligands changes, and the
ligand name is placed in parenthesis.
2 di or bis
5 penta or pentakis
3 tri or tris
6 hexa or hexakis
4 tetra or tetrakis 7 hepta or heptakis
8 octa or octakis
9 nona or nonakis
10 deca or decakis
Nomenclature of Coordination
Compounds
4. Ligands are listed in alphabetical order (ignoring any
prefixes). Most ligands have special names, with all
negatively charged ligands ending in the letter“o”.
Most neutral ligands retain their usual names, with
the following common exceptions:
NH3 ammine
H2O aqua
CO carbonyl
Names of Common Ligands
Formula
BrCO32ClCNHOHO2-
Name
bromo
carbonato
chloro
cyano
hydrido
hydroxo
oxo
Linkage Isomerism
Formula
NO2NO2-
Name
nitrito (via O)
nitro (via N)
Linkage Isomers
Linkage isomers
involve ligands that may
bond via different sites.
In this example, nitro
bonds via nitrogen
(NO2-), and nitrito bonds
via an oxygen (NO2-).
The compounds have
different properties and
colors.
Polydentate Ligands
Formula
NH2CH2CH2NH2
Name
ethylenediamine (en)
Both amines of the
ligand can attach at the
metal forming a ring.
Polydentate Ligands
ethylenediaminetetraacetate (EDTA) EDTA
is a hexadentate ligand.
EDTA
EDTA can wrap
around a metal ion to
coordinate at 6
(octahedral) sites.
Ligands that bind to
more than one site
are called chelating
agents.
Nomenclature of Coordination
Compounds
5.
There are two systems for indicating the oxidation
number of the metal. The more commonly used
system indicates the oxidation number in Roman
numerals in parentheses after the name of the metal.
The other system puts the charge of the
coordination complex in Arabic numbers in
parentheses after the metal.
[Cr(H2O)5Cl]2+ is pentaaquachlorochromium(III)
or, pentaaquachlorochromium(2+).
Nomenclature of Coordination
Compounds
6. Using either system, if the transition metal complex is
negative in charge, the name of the metal ends in ate.
For example, [Pt(NH3)2Cl4]2- is named
diamminetetrachloroplatinate(II).
For metals with Latin names, the negatively charge
complex uses:
ferrate (for Fe)
argentate (for Ag)
plumbate (for Pb) stannate (for Sn)
aurate (for Au)
cuprate (for Cu)
Nomenclature of Coordination
Compounds
7. The complete name of the complex must also indicate
the presence of geometric isomers. Prefixes such as cis,
trans, mer, and fac are used to indicate the relative
positions of similar ligands.
In addition, stereoisomers are also possible with
tetrahedral and octahedral geometries, and optical
isomers are indicated with the prefixes ∆ and Λ.
Nomenclature of Coordination
Compounds
8. Bridging ligands between two metal atoms have the
prefix Ој.
Isomerism
Stereoisomerism
Stereoisomers have the same connectivities
but different spatial arrangements.
In geometric isomers, the ligands have different
spatial arrangements about the metal ion.
Optical isomers are compounds with nonsuperimposable mirror images.
Geometric Isomerism
Geometric isomers differ in the geometric
arrangement of the ligands around the central metal.
Common examples are square planar complexes
such as [Pt(NH3)2Cl2].
Geometric Isomerism
In octahedral complexes, the prefixes cis and trans
are used for complexes of the form [MX4Y2]
Octahedral Complexes
For complexes with the formula [MX3Y3],
there are two spatial arrangements of the
ligands.
Octahedral Complexes
fac stands for facial, and mer stands for
meridian.
Chirality
Both four-coordinate and six-coordinate
complexes exhibit chirality. Chiral molecules
have either no symmetry elements (other than
identity), or only a Cn axis.
Tetrahedral complexes can be chiral in the
same way that organic compounds are: they
may have four different ligands. They may also
have unsymmetrical chelating ligands.
Chirality
Square-planar
complexes can
also be chiral, as
seen in these
compounds of
platinum(II) and
palladium(II).
Optical Isomers
Octahedral complexes containing
polydentate ligands can form optical isomers.
Complexes with three rings, such as [Co(en)3]3+,
can be viewed like a propeller with three blades.
The structure can be either left or right handed,
with non-superimposable mirror images.
Optical Isomers
The upper isomer
is right handed, and the
lower one is left
handed.
Optical Isomers
Optical Isomers
Optical Isomers
Optical Isomers
Optical Isomers
Optical Isomers
The right-handed
isomer requires going
clockwise to get from the
upper triangle to the
lower one. The prefix
for this isomer is ∆.
Optical Isomers
The left-handed
isomer requires going
counterclockwise to get
from the upper triangle
to the lower one. The
prefix for this isomer is
О›.
Optical Isomers
Octahedral complexes with two chelating
ligands and two non-chelating ligands can also
be optically active.
Isomers
пЃ®
Co(III) and ethylenediamine react to form
several products. cis[CoCl2(en)2]+ is violet, and
the trans isomer is green. The reaction also
forms a yellow product, [Co(en)3]3+. Determine
the number of isomers of each of the products.
Label any enantiomers with the proper prefix (∆
or О›).
Isomer Problem
The yellow product is [Co(en)3]+3. It exists
as an enantiomeric pair.
Isomer Problem
The violet product consists of a pair of optical
isomers. The green product is not optically active, as it
has a mirror plane.
Common Coordination Numbers &
Structures
Factors Affecting Coordination
Number
1. The size of the central atom or ion.
2. Steric interactions between bulky ligands such
as P(C6H5)3.
3. The electronic structure of the metal atom or
ion. If the oxidation number is high, the metal
can accept more electrons from the (Lewis base)
ligands. Metals with many d electrons will have
lower coordination numbers.
Coordination Numbers
Coordination numbers of 1, 2 and 3 are
relatively rare. Coordination number 4 leads to
two common structures: tetrahedral or square
planar geometry. Square planar structures are
most commonly seen with d8 metals such as
Ni(II), Pd(II) or Pt(II), or when the ligand is
planar.
5 Coordinate Complexes
5 coordinate complexes with monodentate
ligands are often highly fluxional, so that axial
and equatorial positions interchange. Once such
method of interchange of these positions is via a
Berry pseudorotation.
Berry Pseudorotation
A
A
A
A
A
A
5 Coordinate Complexes
A planar
polydentate ligand,
such as a porphyrin,
may result in square
pyramidal structures,
when a fifth ligand is
added, such as in
myoglobin.
6-Coordinate Complexes
6-coordinate
complexes are usually
octahedral.
Sometimes distortion
from octahedral
geometry occurs if a
bidentate ligand has a
small bite angle.
6-Coordinate Complexes
In extreme cases,
the “staggered”
configuration of the
octahedron can be
distorted into a
trigonal prism.
7- Coordinate Complexes
These are
more common
for f-block
elements, and
include
pentagonal
bipyramids,
capped
octahedrons
and capped
trigonal prisms.
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