Download Transition Metal Complexes: Definitions and Terminology.

2P32 – Principles of Inorganic Chemistry
Dr. M. Pilkington
Lecture 4 - Transition Metal Complexes

Transition Metal Complexes: Definitions
and Terminology.

Isomerism in Transition Metal Complexes:
Structural Isomers and Stereoisomers.
1. Transition Metal Complexes: Definitions and Terminology.
General Convention

The word ligand is derived from the Latin verb ‘ligare’ meaning to
bind.

In a complex
p
we have a Lewis Acid Base interaction:

An arrow is used to show the donation of an electron pair from a
neutral ligand to an acceptor.

A line is used to denote the interaction between an anionic ligand and
the acceptor.

Often however, this convention is ignored and a line to denote both
types of interaction is used.
F example:
For
l
Co3+
Cl
Co3+
Check out this website http://www.chem.purdue.edu/gchelp/cchem/struct2.html
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3+
NH3
H3N
NH3
Co
H3 N
NH3
Review your Acid/Base Interactions
When a Lewis base donates a pair of electrons to a
Lewis acid, a coordinate bond is formed and the resulting
species is an adduct or coordination complex.
NH3
Each N atom donates a pair of electrons to the Co3+ metal ion, i.e. each
NH3 molecule is a Lewis base while the metal ion is the Lewis acid.

We think of the metal to ligand interaction as being essentially covalent,
but in reality this is not entirely true as the character of metal-ligand
interactions varies with the nature of the metal ion and the ligand.

H3 N
NH3
NH3
NH3
H3N
H3N
NH3
NH3
NH3
NH3
In reality the situation is a little more
complex:
NH3
Co
or
Co
H3N
NH3
NH3
NH3
H 3N
or
Co
H 3N
3+
3+
3+
NH3
+
3+
3+
NH3
+
NH3
+
H3 N
NH3
Co3+
+
+
H3N
+NH
NH3
H 3N
Co3NH3
H 3N
NH3
NH3
3
(a)
(b)

Coordinate bonds are formed by lone pair donation from the ligands
the Co3+ centre.

It implies transfer of charge from ligand to metal and figure (a)
shows the resulting charge distribution. This is unrealistic since the
Co3+ centre becomes more negatively charged than would be
unfavorable given its electropositive nature.

At the other extreme, consider bonding in terms of an ionic model
(b) the 3+ charge remains localized on the cobalt and the six NH3
(b),
ligands remain neutral. However this model also does not agree with
experimental studies on this complex. So this model is flawed.

Neither model is appropriate.
2
+
1/2
3+
NH3
1/2 +
+
1/2
H3N
NH3
Co0
1/2 +
H3 N
+
1/2
1/2
+
NH 3
NH3

We have to apply Pauling’s Electroneutrality Principle which states
that the distribution of charge on a molecule or ion is such that the
charge on a single atom is within the range +1 to -1 (ideally close to
zero).

In this case the net charge on the Co3+ metal centre should be close
to zero .

In order to satisfy this the Co3+ ion can accept a total of only 3
electrons from the six ligands, thus giving the charge distribution
above.

This model is actually 50% ionic and 50% covalent.
Co3+
Cl
Co3+

This representation shows that a bridging chloride ion donates two pairs
of electrons to two Co3+ metal ions which are the Lewis acids, accepting
the lone pairs.

In reality,
lit when
h n thinking
thinkin about
b t th
the b
bonding,
ndin th
the formal
f m l charge
h
on
n the
th
chloride ion is not actually -1, which means also that the formal charge
on the two Co3+ metal ions are not strictly +3. either.

What is important is that we have a complex above which has an overall
charge of +5. In order to easily determine the overall charge of the
complex, the above representation is easy to use (3+3-1 = 5).

With respect to thinking about the coordinate bond it does not however
accurately represent the formal charges on the metal ions and the
ligands.

This is analogous to a C-Cl bond in organic chemistry, we write C-Cl but
in reality this does not accurately describe the bonding interaction
since the electrons are not evenly shared and the truth is Cδ+–Clδ-.
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Determination of Formal Oxidation States of Metals
in Coordination Complexes

To figure out the oxidation state or oxidation number of the
central metal atom in a complex is very important
important. Proceed as
follows:

identify the charges on the ligands

look at the total charge on the molecule

֜ charge of ligands + formal oxidation state = total charge

e.g. [FeCl4]2- has 4 Cl- ligands and overall 2- charge, so it must
contain Fe+2 or Fe(II).
Review of Isomerism - Structural Isomers and Stereoisomers.
Isomers – Compounds with the same formula but different
properties that result from different structures. There are two
broad classes of isomers: structural isomers and stereoisomers.
1. Structural isomers have the same molecular formula but
different molecular structures (different connectivities or
different numbers and kinds of chemical bonds.

Organic examples of structural isomers:
CH3OCH3 (dimethylether) and CH3CH2OH (ethanol).
C4H8 cyclobutane
l b t
and
d 11-butene
b t
H2 C
CH2
H2 C
CH2
CH3CH2CH=CH2
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2. Stereoisomers not only have the same formulas but also the
same connectivities of their atoms. The spatial arrangements
of the atoms are different. There are two examples:
i.
Geometric isomers have different spatial arrangement
results
l in different
d ff
geometries (d
(different
ff
b
bond
d angles
l or
different distances between nonbonded atoms, for example).
Organic example: cis- and trans-2-butene, CH3CH=CHCH3
H3C
H
H3C
H
H
H
CH3
cis
2.
CH3
trans
Optical isomers have the same geometrical parameters but
are related as nonsuperimposable mirror images. (In other
words, the molecule or ion is chiral.). Optical isomers get
their names because they are able to rotate a planepolarized light beam to the left or to the right.
Organic example: CHFClI. A carbon atom with four
different groups attached to it has a nonsuperimposable
mirror image.
H
C
F
Cl
H
I
C
I
Cl
F
mirror images non superimposable
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2. Isomerism in Transition Metal Complexes
Structural Isomers
There are many types of structural isomers in transition metal
complexes. We will explore three of them.
g
inside the coordination sphere
p
1. Ionization isomers - Ligands
exchange places with ligands outside the coordination sphere.
Ionization isomers are so-named because they give different ions
when dissolved in water.
Example: There are three compounds with the formula
CrCl3.6H2O. One is violet, one is grey-green, and the third is deep
green.
The violet isomer produces 3 moles of silver chloride upon
reaction with silver nitrate, and does not lose water in a
desiccator.
[Cr(H2O)6]Cl3 (violet)
The grey-green isomer gives 2 moles of silver chloride upon
reaction with silver nitrate, and loses one mole of water when
stored in a desiccator.
[Cr(H2O)5Cl]Cl2.H2O (grey-green)
The deep green isomer gives 1 mole of silver chloride upon
reaction with silver nitrate, and loses two moles of water when
stored in a desiccator.
[Cr(H2O)4Cl2]Cl.2H2O (deep green)
Thus the three ionzation isomers are [Cr(H2O)6]Cl3 (violet),
[Cr(H2O)5Cl]Cl2.H2O (grey-green), and [Cr(H2O)4Cl2]Cl.2H2O
(deep green).
Note that the chloride ions that react with silver nitrate are
the ones not bonded to the chromium(III) ion, and the water
molecules that are lost in a desiccator are the uncoordinated
ones.
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2.
Linkage isomers - Linkage isomers can exist when one or more
ambidentate ligand is bonded to a metal ion.
NO2- An 18 electron system
O
N
O
O
N
O
an ambidentate ligand
A compound with the formula CoCl2(NO2).5NH
) 5NH3 has two isomers,
isomers
one yellow and one red.
Each precipitates two moles of silver chloride, therefore both
chloride ions are outside the cobalt(III) coordination sphere.
Neither has an aqueous solution that is basic to pH paper,
therefore all the ammonias are bonded to cobalt (III).
The obvious possibility is that the ambidentate nitrite group is
differently bonded in these two complexes: [Co(NH3)5NO2]Cl2 and
[Co(NH3)5ONO]Cl2.
Today, we would assign the structures on the basis of infrared
spectra: N- and O-bonded nitrite have different N-O stretching
frequencies.
The Ambidentate Nitrite ion NO2-
Mn+ Nitro linkage
Mn+ Nitrito linkage
A resonance hybrid, showing the N-O bonds
iin th
the nitrite
it it iion h
have a bond
b d order
d of
f about
b t
1.5, leaving most of the single negative
charge shared between the terminal oxygen
atoms
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3.
Coordination isomers - involve ligand exchange between
coordination spheres of two metal ions that are part of the
same compound.

[Pt(NH3)4]2+ [PtCl4]2- and [Pt(NH3)3Cl]+[Pt(NH3)Cl3]-

Formula, both atoms contain Pt2Cl4(NH3)4

[Pt(NH3)2Cl2] has the same ratio of atoms, but does not have
the same overall formula; hence it is not a coordination isomer
of the above compounds.
2. Isomerism in Transition Metal Complexes
Stereoisomers
i.
Geometric Isomers are found in square planar and octahedral
complexes.
Examples
p
for square
q
planar
p
coordination are the cis- and
trans-isomers of diamminedichloroplatinum(II):
Note the convention of drawing a square with the metal ion in
the center and the ligands at the corners of the square .
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An example of geometric isomers in octahedral complexes are
the cis- and trans-isomers of the
tetraamminedichlorocobalt(III) ion:
Note that there are several different ways to represent an
octahedrally coordinated metal ion; which way you choose
depends
p
on what you
y are trying
y g to show.
Isomers – compounds with same
molecular formula but different properties
Structural Isomers -Same
molecular formula but different
connectivities –different numbers
and kinds of chemical bonds
1. Ionization
2. Linkage
3. Coordination
Stereoisomers – same
connectivities, different
spatial arrangement of atoms
Geometric
cis/trans
Optical
(enantiomers)
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