Coordination Chemistry 3 Flashcards
cft: overview
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The crystal field theory (CFT) is an electrostatic model which considers
the metal-ligand bond to be ionic arising purely from electrostatic
interactions between the metal ion and the ligand. Ligands are treated
as point charges in case of anions or point dipoles in case of neutral
molecules. The five d orbitals in an isolated gaseous metal atom/ion
have same energy, i.e., they are degenerate. This degeneracy is
maintained if a spherically symmetrical field of negative charges
surrounds the metal atom/ion. However, when this negative field is
due to ligands (either anions or the negative ends of dipolar molecules
like NH3 and H2O) in a complex, it becomes asymmetrical and the
degeneracy of the d orbitals is lifted. It results in splitting of the d
orbitals. The pattern of splitting depends upon the nature of the crystal
field.
basis of crystal field splitting in octahedral complexes
(i)In an octahedral coordination entity with six ligands surrounding
the metal atom/ion, there will be repulsion between the electrons in
metal d orbitals and the electrons (or negative charges) of the ligands.
(ii)Such a repulsion is more when the metal d orbital is directed towards the ligand than when it is away from the ligand.
(iii) Thus, the dx2-y2 and dz2 orbitals which point towards the axes along the direction of
the ligand will experience more repulsion and will be raised in energy; and the dxy, dyz and dxz orbitals which are directed between
the axes will be lowered in energy relative to the average energy in
the spherical crystal field.
iv) Thus, the degeneracy of the d orbitals
has been removed due to ligand electron-metal electron repulsions
in the octahedral complex to yield three orbitals of lower energy, t2g
set and two orbitals of higher energy, eg set.
what is crystal field splitting
This splitting of the degenerate levels due to the
presence of ligands in a
definite geometry is termed as
crystal field splitting and
the energy separation is
denoted by Do
(the subscript
o is for octahedral) (Fig.5.8).
Thus, the energy of the two eg
orbitals will increase by (3/5)
Do and that of the three t2g will
decrease by (2/5)Do
.
what does crystal field splitting energy depends upon
The crystal field splitting,
Do
, depends upon the field
produced by the ligand and
charge on the metal ion. Some
ligands are able to produce
strong fields in which case, the
splitting will be large whereas
others produce weak fields
and consequently result in
small splitting of d orbitals.
spectrochemical series
In general, ligands can be arranged in a series in the order of increasing
field strength.
It is an experimentally determined series based on the absorption of light by complexes with different ligands.
I-< Br-< SCN-<Cl-<S2-<F-< OH-<C2O42-<H2O<NCS-<edta4-<nh3<en<cn-<co
electronic configuration
the single d electron occupies one of the lower energy t2g orbitals. In
d
2
and d
3
coordination entities, the d electrons occupy the t2g orbitals
singly in accordance with the Hund’s rule.
For d4 ions, two possible
patterns of electron distribution arise: (i) the fourth electron could
either enter the t2g level and pair with an existing electron, or (ii) it
could avoid paying the price of the pairing energy by occupying the
eg
level.
when do the 2 cases of d4 electron occurs
Which of these possibilities occurs, depends on the relative
magnitude of the crystal field splitting, Do
and the pairing energy, P
(P represents the energy required for electron pairing in a single
orbital). The two options are:
(i) If Do < P, the fourth electron enters one of the eg orbitals giving the configuration t2g³eg¹ . Ligands for which Do < P are known as weak field ligands and form high spin complexes.
(ii) If Do> P, it becomes more energetically favourable for the fourth electron to occupy a t2g orbital with configuration t2g⁴
eg⁰ . Ligands which produce this effect are known as strong field ligands and form low spin complexes.
Calculations show that d4 to d7 coordination entities are more stable for strong field as compared to weak field cases.
Crystal field splitting in tetrahedral coordination entities
In tetrahedral coordination entity formation,
the d orbital splitting (Fig. 5.9) is inverted
and is smaller as compared to the octahedral
field splitting. For the same metal, the same
ligands and metal-ligand distances, it can
be shown that Dt
= (4/9) D0
. Consequently,
the orbital splitting energies are not
sufficiently large for forcing pairing and,
therefore, low spin configurations are rarely
observed. The ‘g’ subscript is used for the
octahedral and square planar complexes
which have centre of symmetry. Since
tetrahedral complexes lack symmetry, ‘g’
subscript is not used with energy levels.
Colour in
Coordination
Compounds
This means that some of the visible spectrum is being removed from
white light as it passes through the sample, so the light that emerges
is no longer white. The colour of the complex is complementary to
that which is absorbed. The complementary colour is the colour
generated from the wavelength left over; if green light is absorbed by
the complex, it appears red.
why is [Ti(H2O)6]3+ violet in colour
This is an octahedral complex
where the single electron (Ti
3+ is a 3d1 system) in the metal d orbital is
in the t2g level in the ground state of the complex. The next higher state
available for the electron is the empty eg
level.
If light corresponding to
the energy of blue-green region is absorbed by the complex, it would
excite the electron from t2g level to the eg
level (t2g1eg0 ® t2g0eg1).
Consequently, the complex appears violet in colour (Fig. 5.10). The
crystal field theory attributes the colour of the coordination compounds
to d-d transition of the electron.
when is [Ti(H2O)6]3+ colourless
It is important to note that
in the absence of ligand,
crystal field splitting does
not occur and hence the
substance is colourless. For
example, removal of water
from [Ti(H2O)6
]Cl3 on heating
renders it colourless.
Similarly, anhydrous CuSO4
is white, but CuSO4
.5H2O is
blue in colour
[Ni(H2O)6]2+ (aq) + en (aq) = [Ni(H2O)4(en)]2(aq)+ 2H2O
green pale blue
[Ni(H2O)4 (en)]2+(aq) +en(aq) =[Ni(H2O)2(en)2]2+(aq)
blue/purple
[Ni(H2O)2(en)2]2+(aq) + en (aq) = [Ni(en)3]2+(aq) +2H2O
violet
Limitations
of Crystal
Field
Theory
However, from the assumptions that the ligands are point charges, it follows that anionic ligands should exert the greatest
splitting effect. The anionic ligands actually are found at the low end
of the spectrochemical series.
Further, it does not take into account
the covalent character of bonding between the ligand and the central
atom. These are some of the weaknesses of CFT, which are explained
by ligand field theory (LFT) and molecular orbital theory
