Coordination Chemistry Flashcards
what are isomers
Isomers are two or more compounds that have the same chemical
formula but a different arrangement of atoms. Because of the different
arrangement of atoms, they differ in one or more physical or chemical
properties. Two principal types of isomerism are known among
coordination compounds.
(a) Stereoisomerism
(i) Geometrical isomerism (ii) Optical isomerism
(b) Structural isomerism
(i) Linkage isomerism (ii) Coordination isomerism
(iii) Ionisation isomerism (iv) Solvate isomerism
stereoisomers and structural isomers
Stereoisomers have the same chemical formula and chemical
bonds but they have different spatial arrangement. Structural isomers
have different bonds
Geometric Isomerism in square planar structures
This type of isomerism arises in heteroleptic
complexes due to different possible geometric
arrangements of the ligands.
In a square planar complex of
formula [MX2L2
] (X and L are unidentate), the
two ligands X may be arranged adjacent to each
other in a cis isomer, or opposite to each other
in a trans isomer as depicted in Fig. 5.2.
Other square planar complex of the type
MABXL (where A, B, X, L are unidentates)
shows three isomers-two cis and one trans.
geometric isomerism in octahedral structures
octahedral complexes of formula [MX2L4
] in
which the two ligands X may be oriented cis or
trans to each other
This type of isomerism also
arises when didentate ligands
L – L [e.g., NH2 CH2 CH2 NH2 (en)]
are present in complexes of formula
[MX2
(L– L)2]
fac and mer isomer
Another type of geometrical
isomerism occurs in octahedral
coordination entities of the type
[Ma3b3] like [Co(NH3)3(NO2)3]. If
three donor atoms of the same
ligands occupy adjacent positions
at the corners of an octahedral
face, we have the facial (fac)
isomer. When the positions are
around the meridian of the
octahedron, we get the meridional
(mer) isomer
Why is geometrical isomerism not possible in tetrahedral complexes
having two different types of unidentate ligands coordinated with
the central metal ion ?
Tetrahedral complexes do not show geometrical isomerism because
the relative positions of the unidentate ligands attached to the central
metal atom are the same with respect to each other.
optical isomers
Optical isomers are mirror images that
cannot be superimposed on one
another. These are called as
enantiomers. The molecules or ions
that cannot be superimposed are
called chiral. The two forms are called
dextro (d) and laevo (l) depending
upon the direction they rotate the
plane of polarised light in a
polarimeter (d rotates to the right, l to
the left).
Optical isomerism is common
in octahedral complexes involving
1,2,3 didentate ligands (Fig. 5.6).
In a coordination
entity of the type
[PtCl2(en)2]
2+, only the
cis-isomer shows optical
activity
Linkage Isomerism
Linkage isomerism arises in a coordination compound containing
ambidentate ligand. A simple example is provided by complexes
containing the thiocyanate ligand, NCS–
, which may bind through the
nitrogen to give M–NCS or through sulphur to give M–SCN.
Jørgensen discovered such behaviour in the complex [Co(NH3)5(NO2)]Cl2, which is obtained as the red form, in which the nitrite ligand is bound through oxygen (–ONO), and as the yellow form, in which the nitrite ligand is bound through nitrogen (–NO2).
Coordination
Isomerism
This type of isomerism arises from the interchange of ligands between
cationic and anionic entities of different metal ions present in a complex.
An example is provided by [Co(NH3)6][Cr(CN)6], in which the NH3 ligands are bound to Co3+ and the CN– ligands to Cr3+. In its coordination isomer [Cr(NH3)6][Co(CN)6], the NH3 ligands are bound to Cr3+ and the CN– ligands to Co3+
Ionisation Isomerism
This form of isomerism arises when the counter ion in a complex salt
is itself a potential ligand and can displace a ligand which can then
become the counter ion. An example is provided by the ionisation
isomers [Co(NH3)5(SO4)]Br and [Co(NH3)5Br]SO4
.
Solvate
Isomerism
This form of isomerism is known as ‘hydrate isomerism’ in case where
water is involved as a solvent. This is similar to ionisation isomerism.
Solvate isomers differ by whether or not a solvent molecule is directly
bonded to the metal ion or merely present as free solvent moleculesin
the crystal lattice. An example is provided by the aqua
complex [Cr(H2O)6
]Cl3
(violet) and its solvate isomer [Cr(H2O)5Cl]Cl2
.H2O
(grey-green).
disadvantages of werners theory
But his theory could not answer basic questions like:
(i) Why only certain elements possess the remarkable property of
forming coordination compounds?
(ii) Why the bonds in coordination compounds have directional
properties?
(iii) Why coordination compounds have characteristic magnetic and
optical properties?
Many approaches have been put forth to explain the nature of
bonding in coordination compounds viz. Valence Bond Theory (VBT),
Crystal Field Theory (CFT), Ligand Field Theory (LFT) and Molecular
Orbital Theory (MOT).
vbt
According to this theory, the metal atom or ion under the influence of
ligands can use its (n-1)d, ns, np or ns, np, nd orbitals for hybridisation
to yield a set of equivalent orbitals of definite geometry such as octahedral,
tetrahedral, square planar and so on (Table 5.2). These hybridised orbitals
are allowed to overlap with ligand orbitals that can donate electron pairs
for bonding.
It is important to note that the hybrid orbitals do not actually exist.
In fact, hybridisation is a mathematical manipulation of wave equation
for the atomic orbitals involved.
inner orbital/ low spin/ spin paired complex
In the formation
of this complex, since the inner d orbital (3d) is used in hybridisation,
the complex, [Co(NH3)6]
3+ is called an inner orbital or low spin or spin
paired complex
outer orbital/ high spin/ spin free
The paramagnetic octahedral complex, [CoF6]
3–
uses
outer orbital (4d ) in hybridisation (sp
3
d
2
). It is thus called outer orbital
or high spin or spin free complex. Thus:
