The intense colour of the latter complex is strongly suggestive of a
charge transfer transition and since the metal is already fully
reduced (zero oxidation state), it is highly likely that this
involves a MLCT transition. The π* levels of the bipy or CO ligands are possible acceptors but
the fact that Fe(CO)5 doesn’t show this colour
suggests that it is the bipy π* levels that are involved in Fe(bipy)(CO)3. Since there should be
MLCT transitions to the CO π* levels as well, we assume that the lack of
colour for Fe(CO)5 means that these
transitions fall in the UV rather than the visible.
Wednesday, August 28, 2019
Why all the tetrahedral Complexes are high spin Complexes ?
The magnitude of crystal field splitting energy (CFSE) in tetrahedral Complexes is quite small and it is always less
than the pairing energy. Due to this reason pairing of electron is energetically
unfavorable. Thus all the tetrahedral Complexes are high spin Complexes. In fact no tetrahedral Complex
with low spin has been found to exist.
Tuesday, August 27, 2019
FACTOR'S AFFECTING STABILITY OF COMPLEXES:
(1) Charge on central metal cation:
In general stability of
Complexes is directly proportional to the magnitude of charge on central metal
atom. Thus Complexes of Fe +3 are more stable than Fe+2
(2) Size of central metal
cation:
As size of the central atom ion
decreases the stability of the Complex increases. This is applicable when
Oxidation state of central metal ion is same in all the cases. For example Zn+2
< Cu+2 < Ni+2 < Co+2 < Fe+2
< Mn+2
(3) Nature of the ligands:
The size and charge of ligands
is also an important factor of in deciding the stability of complexes.
(i) If the Ligand is smaller, it can approach
the central metal ion more closely forming a stable bond.
(ii) High charge ligand will form a strong bond.
Thus high charge and small size of ligands leads to Formation of stable
Complexes.
(iii) Charge density of the ligand= charge/size. More the charge density more is the
stability of the Complex. For example fluoride (F- ) will form more
stable Complexes and Iodide (I-) forms least stable Complexes.
(4) Chelating effects:
Formation of five and six
membered chelate from polydentate ligands enhance the stability of Complexes in
comparison to monodentate ligands. This is called chelate effect of chelation.
In general, the more number of chelate rings in the Complex, the more will be
stability of the Complex.
(5) Formation constant of
Complexes:
Stronger is the metal- ligand
bond, less is the dissociation of Complex ion in the Solution and hence greater
is the stability of complex. Thus the larger the numerical value of formation
constant, the thermodynamically more stable is the complex.
Sunday, August 25, 2019
What is d-d transition in complexes and explain colour of complex by d-d transition ?
Most
of the transition metal compounds are coloured both in the solid state and
in aqueous solution. This is because of the presence of incompletely
filled d-orbitals. When a transition metal compound is formed the degenerate d-orbitals of the metal split into
two sets, one having three orbitals dxy, dyz and dxz called t2g orbitals with lower energy and the other having two
orbitals dx2 –y2 and dz2 called eg orbitals with slightly higher energy in an octahedral field.
This is called crystal field splitting When white light falls on these
compounds, some wavelength is absorbed for promotion of electrons from one
set of lower energy orbitals to another set of
slightly
higher energy within the same d-subshell. This is
called d-d transition. The remainder light is reflected which has a particular
colour.
The colours of some 3d metal
ions:
SN
|
d-configuration
|
Examples with colour
|
1
|
d0 (No d-d transition)
|
|
2
|
d1
|
Ti3+
(3d1) Purple, V+4(3d1)
Blue
|
3
|
d2
|
V+3(3d1)
Green
|
4
|
d3
|
Cr3+
(3d3) Violet green
|
5
|
d4
|
Mn+3(3d4)
Violet , Cr2+ (3d4) Blue
|
6
|
d5
|
Mn+2(3d5)
Pink, Fe+3(3d5) Yellow
|
7
|
d6
|
Fe+2(3d6)
Brown , Co+2(3d6) Green ,
|
8
|
d7
|
|
9
|
d8
|
Ni+2(3d8)
Green
|
10
|
d9
|
Cu+2(3d9)
Blue
|
10
|
d10
|
Sc+3(3d0)
colourless
|
ILLUSTRATION (1): The mechanism
of light absorption in coordination compounds is that photons of appropriate
energy can excite the coordination entity from its ground state to an excited
state. Consider [Ti(H2O)6]3+
In which Ti(+3) ion has one electron in d sub shell ( in
lower energy t2g
d- orbital) . In aqueous solution, [Ti(H2O)6]3+. Appear as purple due to
the absorption of light from visible range ( green and yellow portion)
resulting d-d transition ( electron jump
from t2g level to eg level) as result complex has complementary ie purple .
The
variety of color among transition metal complexes has long fascinated the
chemists.
ILLUSTRATION (2) : Aqueous solutions of [Fe(H2O)6]3+
are red, [Co(H2O)6]2+ are pink, [Ni(H2O)6]2+ are green, [Cu(H2O)6]2+ are blue and [Zn(H2O)6]2+
are colorless. Although the octahedral [Co(H2O)6]2+
are pink, those of tetrahedral [CoCl4]2- are blue. The green color of [Ni(H2O)6]2+
turns blue when ammonia is added to give [Ni(NH3)6]2+. Many of these facts
can be rationalized from CFT.
Related Question:
(1) Although both [Mn(H2O)6]2+ and [FeF6]3- have a d5 configuration and high-spin complexes. But the dilute solutions of Mn2+ and Fe +3 complexes are therefore colorless. Why?
Related Question:
(1) Although both [Mn(H2O)6]2+ and [FeF6]3- have a d5 configuration and high-spin complexes. But the dilute solutions of Mn2+ and Fe +3 complexes are therefore colorless. Why?
Factor's affecting magnitude of crystal field stabilisation energy (CFSE): MAGNITUDE OF CFSE:
CRYSTAL FIELD STABILISATION ENERGY(CFSE)
Magnitude of CFSE depends upon the following factors.
(1) Nature of central metal cation: the the value of CFSE depends other following factors of central metal cation as given as
(a) For the Complex having same geometry and same ligands but having different numbers of d-electrons then CFSE decrease on increasing number of d-electrons in the central metal cation.
(1) Nature of central metal cation: the the value of CFSE depends other following factors of central metal cation as given as
(a) For the Complex having same geometry and same ligands but having different numbers of d-electrons then CFSE decrease on increasing number of d-electrons in the central metal cation.
(b) When numbers
of d-electrons are same
then CFSE increasing on increasing Oxidation number.
(c) For same Ligand, Oxidation state, same
d-electrons, CFSE increasing on increasing principle quantum Number of d-
orbitals like 3d
< 4d < 5d etc.
Thus the elements of the second and third transition
series have greater tendency to form low spin complexes than the first
transition series. It is possible to arrange the metals according to a
spectrochemical series as well. The approximate order is
(2) Nature of ligands: the magnitude of
CFSE varies from stronger ligand to weaker ligands it meant CFSE
increasing on increasing of splitting power of ligands and decreasing on
decreasing of splitting power of ligands.
Splitting power of ligands decide according to spectrochemical series.
(3) Geometry of the Complex: the value of CFSE will change with geometry of Complexes. It is estimated that CFSE of tetrahedral Complexes is approximately 50% as large as that of octahedral Complexes.
Illustrative example (1):
Which of the Complex of the following pairs has the largest value of CFSE?
(1) [Co(CN)6]3- and [Co(NH3)6]3+
(2) [Co(NH3)6]3+ and [CoF6]3-
(3) [Co(H2O)6]3+ and [Rh(H2O)6]3+
(4) [Co(H2O)6]2+ and [Co(H2O)6]3+
(1) [Co(CN)6]3- and [Co(NH3)6]3+
(2) [Co(NH3)6]3+ and [CoF6]3-
(3) [Co(H2O)6]3+ and [Rh(H2O)6]3+
(4) [Co(H2O)6]2+ and [Co(H2O)6]3+
SOLUTION:
(1) CN is the stronger ligand than NH3 therefore CFSE of [Co(CN)6]3- will be more than [Co(NH3)6]3+
(2) NH3 is stronger ligand than F therefore CFSE of [Co(NH3)6]3+ will be more than [CoF6]3- .
(3) Co belong to 3d series whereas The Rh belong to 4d series. More the value of n more is CFSE therefore CFSE of [Rh(H2O)6]3+ is more than [Co(H2O)6]3+ .
(4) Oxidation number of Co in [Co(H2O)6]3+ is more than the Oxidation number of [Co(H2O)6]2+ therefore, CFSE of [Co(H2O)6]3+ is more than [Co(H2O)6]2+ .
Related questions:
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