CH437Electronic Spectroscopy 1

8/13/2019 CH437Electronic Spectroscopy 1

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Electronic Spectra of Transition Metal ComplexesElectronic Spectra of Transition Metal Complexes

Spectra arises because,

1.1. electrons may be promoted from one energy level to another.electrons may be promoted from one energy level to another.

2.2. Such electronic transition require high energy.Such electronic transition require high energy.

3.3. During electronic transitions low energyDuring electronic transitions low energy vibrationalvibrational and rotational transition willand rotational transition willalways occuralways occur

4.4. In electronic spectra such transitions are too close in energy to be resolved intoIn electronic spectra such transitions are too close in energy to be resolved into

separate absorption bands.separate absorption bands.

5.5. But they cause broadening of absorption bands in dBut they cause broadening of absorption bands in d--d spectrad spectra

6.6. BandBand--widths are in the order of 1000widths are in the order of 1000--3000 cm3000 cm--11..

Spectrum of a coloured solution may be measured quite easily using aSpectrum of a coloured solution may be measured quite easily using a

spectrophotometerspectrophotometerMonochromatic beam of light is passed through a solution on to a photo electric cell.Monochromatic beam of light is passed through a solution on to a photo electric cell.

Amount of light absorbed at any particular frequency can read off, or a whole frequencyAmount of light absorbed at any particular frequency can read off, or a whole frequency

range can be scanned.range can be scanned.


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Absorbance is plotted,

The absorbance A is called optical density which is give by A = log ( )

Io

I

Io = Intensity of the original beam of light

I = Intensity after passing through the solution

the molar absorption coefficient (molar absorptivity coefficient)

calculated from A

= A/Cl C = concentration of the solution in molesl = path length in cms (1 cm long)


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.. ..

AA ,, ,, ,, ..

() : () : . . . . = = 1 1

. .

.. .. 2200 1111

+1, +1, = 5000= 500010000 10000 11 11

= 0 ( = 0 ( = 5= 510 10 11 11))

: :

, .., .. = 0 = 0

( ())


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dd--d transitions are forbidden (d transitions are forbidden (LaporteLaporte forbidden) asforbidden) as l = 0l = 0

Low absorbance (Low absorbance ( = 5= 5--10 l mol10 l mol--11 cmcm--11))

Because of a slight relaxation inBecause of a slight relaxation in LaporteLaporte rule, drule, d--d transitions are observed.d transitions are observed.

When transition metal forms a complex, M is surrounded by LWhen transition metal forms a complex, M is surrounded by L ligandsligands..

Mixing of dMixing of d-- and pand p--orbitalsorbitals may occur and as a result, they are no longer true dmay occur and as a result, they are no longer true d--ddin naturein nature

Mixing is common when complexes lack centre of symmetryMixing is common when complexes lack centre of symmetry

Intensity (Intensity ( value) of absorption increases if the mixing is more effective.value) of absorption increases if the mixing is more effective.

e.g. [MnBr4]2- is tetrahedral

[Co(NH3)5Cl]2+ is octahedral with unsymmetrical substitution, are coloured

e.g. [Co(NH3)6]3+ or [Cu(H2O)6]

2+ have center of symmetry, no mixing of p & d-

orbitals, are not coloured

However, the M-L bond vibrates so that the ligand spends an appreciable amount of time out of

their centrosymmetric equilibrium position, as a result, small amount of mixing occurs and low

intensity transitions are observed.


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Many MnMany Mn2+2+ are offare off--white or palewhite or pale--fleshflesh colouredcoloured and the intensity is only 1/100and the intensity is only 1/100thth ofof

that of spinthat of spin--allowed transitions.allowed transitions.

SpinSpin--forbidden transitions are weak and can be ignored.forbidden transitions are weak and can be ignored.

Thus analyses of spectra of transitions metal complexes are greatly simplified byThus analyses of spectra of transitions metal complexes are greatly simplified by

ignoring all spinignoring all spin--forbidden transitions.forbidden transitions.

Thus for dThus for d22 only terms that are need to be considered areonly terms that are need to be considered areGround StateGround State 33F and the excited stateF and the excited state 33P.P.

Splitting of electronic energy levels and spectroscopic statesSplitting of electronic energy levels and spectroscopic states

ss are symmetrically spherical and unaffected in an Oh fieldare symmetrically spherical and unaffected in an Oh field

pp orbitalsorbitals are directional and are affected by an Oh field (all are affectedare directional and are affected by an Oh field (all are affected

equally and no splitting)equally and no splitting)

eg

t2g

d 10Dq

eg dooubly degenerate 6Dq

triply degenerate 4Dqt2g


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AA

**

A

A

10000 62

500 4

= B,

8 10 (2)63+, (2)6

3+

**

4 B42

0.02 (2)62+


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For d1 ground state is a 2D state; T2g and Eg are spectroscopic states

Eg

T2g

+6

-4

D

A set of f orbitals will split into three levels in an octahedral field

A2 Single

T2g

Triply degenerate

F

T1g

12 Dq

2 Dq

6 Dq


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Spectra of dSpectra of d11 and dand d99 ionsions

In a free gaseous metal ion, dIn a free gaseous metal ion, d orbitalsorbitals degenerate anddegenerate and no dno d--d transitionsd transitions areare

observed.observed.

In a complex, degeneracy is lost and splits intoIn a complex, degeneracy is lost and splits into tt2g2g ((tt22) and) and eegg((ee))

Let us consider [TiCl[TiCl66]]33-- oror [Ti(H[Ti(H22O)O)66]]3+3+ dd11 systemsystem

20300 20300 11AA

BB

0 3000 2000 1000

BB

AA

((11))((22))663+3+

o

E

Magnitude ofMagnitude of oo depends on the nature of the ligands and affects the energy of electronic transitions and hencedepends on the nature of the ligands and affects the energy of electronic transitions and hence

the frequency of absorption maximthe frequency of absorption maxim


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6633

13,000 13,000 11

6633 18,900 18,900 11

((22))663+3+ 20,300 20,300 11

()()6633 22,300 22,300 11

Extent of splitting is related to theExtent of splitting is related to the ligandligand positions in thepositions in the spectrochemicalspectrochemical seriesseries

Let us consider a dLet us consider a d99 systemsystem

Let us considerLet us consider dd99 system: e.g.system: e.g. [Cu(H[Cu(H22O)O)66]]2+2+

Splitting of dSplitting of d--orbitalsorbitals is similar to dis similar to d11 casecase

In dIn d11, one electron is there in t, one electron is there in t2g2g levellevel In dIn d99, one hole is there in, one hole is there in eegg levellevel

In dIn d11, promotion of one electron from T, promotion of one electron from T2g2g EEgg, whilst in, whilst in dd99 itit is simpler tois simpler to

consider the promotion of an electron as the transfer of a hole fromconsider the promotion of an electron as the transfer of a hole from EEgg TT2g2g


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Eg

T2g

6Dq

4 Dq2D

For dFor d99 inverse of dinverse of d11 energy diagram holds goodenergy diagram holds good

dd11and dand d99 in Tetrahedral Field:in Tetrahedral Field: Exactly reverse of that of Oh fieldExactly reverse of that of Oh field

T

E

2D


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Octahedral High Spin complexes

d1 d6 d4 d9

Tetrahedral Hi h S in com lexes

d1 d6 d4 d9

TerahedralTerahedral complexes are always highcomplexes are always high--spinspin is it true???is it true???


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dd11 & d& d66; d; d44 & d& d99 OctahedralOctahedral

dd44 & d& d99; d; d11 & d& d66 TetrahedralTetrahedral

All these cases can be combined into a single diagram calledAll these cases can be combined into a single diagram called OrgelOrgel diagram,diagram,which describes the qualitative way of the effect of electronic configuration withwhich describes the qualitative way of the effect of electronic configuration with

i)i) One electronOne electron

ii)ii) One more electron than a halfOne more electron than a half--filled shellfilled shell

iii)iii) One less electron than a fullOne less electron than a full--shellshell

iv)iv) One less than a halfOne less than a half--filled shellfilled shell

EEgg (E) d(E) d11 & d& d66 OctahedralOctahedral

TT2g2g (T(T22))dd11 & d& d66

TT2g2g (T(T22) d) d44 & d& d99 TetrahedralTetrahedralEEgg (E)(E)

TetrahedralTetrahedral

dd44 & d& d99

OctahedralOctahedral

00IncreasingIncreasing ligandligand fieldfield


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Spectra of dSpectra of d22 and dand d88 ionsions

In an octahedral field tIn an octahedral field t2g2g22eegg00 tt2g2g11eegg11

Two possibilities:Two possibilities:

Electron may be promoted fromElectron may be promoted from

ddxzxz oror ddyzyz to dto dzz2 or d2 or dxx22--yy22

Less energy is needed to promote anLess energy is needed to promote anelectron to delectron to dzz2 than d2 than dxx22--yy22

((ddxyxy))11(d(dzz2)2)

11 ((ddxyxy))11(d(dxx22--yy2)2)

11

less energyless energy more energy for transitionmore energy for transition

Electrons are spread aroundElectrons are spread around

in all three directions x, y & zin all three directions x, y & z

reducing the electronreducing the electron--electronelectron

repulsionrepulsion

Electrons are confinedElectrons are confined xyxy

plane, as a result moreplane, as a result more

electronelectron--electron repulsionelectron repulsion

In both the cases electrons are promoted and another high energy state will beIn both the cases electrons are promoted and another high energy state will be

formed.formed.

Thus four energy levels will be thereThus four energy levels will be there


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For dFor d22: G.S. Term: G.S. Term 33F and four excited states:F and four excited states:

33P,P, 11G,G, 11D &D & 11SS

Ground State contains 2 electrons with parallel spins.Ground State contains 2 electrons with parallel spins.

But theBut the11

G,G,11

D &D &11

S states contain electrons withS states contain electrons withopposite spins and are ignored.opposite spins and are ignored.

33F andF and 33P can have transitionsP can have transitions

pp--orbitalsorbitals are not split,are not split, 33FF AA2g2g + T+ T1g1g + T+ T2g2g

Thus three transitions are possibleThus three transitions are possible


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1.21.2 oo

0.20.2 oo

Let us consider [V(H[V(H22O)O)66]]3+3+ dd22 systemsystem

10

5

3T1g3T1g(P)

3T1g3A2g

3T1g3T2g

olarabsorbance

--0.80.8 oo40000 30000 20000 10000

Electronic spectrum of [V(H2O)6]3+ : d2system

cm-1

Ligand field strength of HLigand field strength of H22O results in transitions occurring close to the crossO results in transitions occurring close to the cross--overover

point betweenpoint between 33TT1g1g(P)(P) andand33TT2g2g(F) and they are not resolved.(F) and they are not resolved.

VV3+3+ ion with three different ligands will show three distinct peaks.ion with three different ligands will show three distinct peaks.


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In case of NiIn case of Ni2+2+, a d, a d88 system with two holes insystem with two holes in eegg

Promoting one or two electrons toPromoting one or two electrons to eegg means transferring themeans transferring thehole(s) to thole(s) to t2g2g levellevel

33P is not split andP is not split and 33F is split into three states and will be invertedF is split into three states and will be inverted

33

AA2g2g will be the Ground State Termwill be the Ground State TermSimilarly, dSimilarly, d77 is similar to dis similar to d22 and dand d33 is similar to dis similar to d88 in an octahedralin an octahedral

environmentenvironment

CrCr3+3+

, a d, a d33

system is expected to show three peakssystem is expected to show three peaks

All these cases can be combined into a single diagram calledAll these cases can be combined into a single diagram called OrgelOrgel diagram,diagram,

which describes the qualitative way of the effect of electronic configuration withwhich describes the qualitative way of the effect of electronic configuration with

i)i)two electrontwo electron

ii)ii)two more electron than a halftwo more electron than a half--filled shellfilled shell

iii)iii)two less electron than a fulltwo less electron than a full--shellshell

iv)iv)two less than a halftwo less than a half--filled shellfilled shell


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Octahedral High Spin complexes

d2 d8 d3 d7

Tetrahedral High Spin complexes

d3 d7 d8 d2


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dd22 octahedraloctahedral33TT1g1g (P)(P)

dd22

tetrahedraltetrahedral

dd33

dd77tetrahedraltetrahedral

33TT1g1g (F)(F)

33TT2g2g

dd77 octahedraloctahedral

dd33 tetraahedraltetraahedral33TT2g2g

33AA2g2g

33TT1g1g (P)(P)

33FF

33PP

Combined Orgel diagram

dd88 tetrahedraltetrahedral


dd88

octahedraloctahedral33AA2g2g

33TT1g1g (F)(F)


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10

53A2g

3T1g(P) 3A2g3T1g(F)

3

A2g3

T2g(F)

Molarabsorbance

40000 30000 20000 10000

Electronic spectrum of [Ni(H2O)6]2+ : d8system

cm-1


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dd22 octahedraloctahedral33TT1g1g (P)(P)

dd22

tetrahedraltetrahedral

dd33

dd77tetrahedraltetrahedral

33TT1g1g (F)(F)

33TT2g2g

dd77 octahedraloctahedral

dd33 tetraahedraltetraahedral33TT2g2g

33AA2g2g

33TT1g1g (P)(P)

33FF

33PP

Combined Orgel diagram

dd88 tetrahedraltetrahedral


dd88

octahedraloctahedral33AA2g2g

33TT1g1g (F)(F)


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We have twoWe have two 33TT1g1g states one each forstates one each for33P andP and 33F stateF state

Both TBoth T1g1g states are curved, because they have the same symmetry, andstates are curved, because they have the same symmetry, and

they interact with each otherthey interact with each other

InterelectronicInterelectronic repulsions lowers the energy of the lower states andrepulsions lowers the energy of the lower states andincreases the energy of the higher state.increases the energy of the higher state.

levels are close in energylevels are close in energy

If the lines have been straight, they would cross each other which impliesIf the lines have been straight, they would cross each other which implies

that at crossthat at cross--over two electrons in one atom have the same symmetry andover two electrons in one atom have the same symmetry and

the same energythe same energy

This is impossible; prohibited by nonThis is impossible; prohibited by non--crossing rule.crossing rule.

State of same symmetry cannot cross each otherState of same symmetry cannot cross each other


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The mixing or interelectronic repulsion which causes the bending of the linesThe mixing or interelectronic repulsion which causes the bending of the lines

is expressed by Racah Parameters B and C.is expressed by Racah Parameters B and C.

B and C can be calculated from liner combination of exchange intetrals andB and C can be calculated from liner combination of exchange intetrals and

Coulomb integralsCoulomb integrals

But they are obtained emperically from the spectra of the free ions.But they are obtained emperically from the spectra of the free ions.

noo mixing

with mixing

4P

4T1g(P)

33

The splitting of the

4

P,

4

F states in Cr

3+

4F

15B4T1g (F)

4T2g

4A2g

6Dq 8Dq

10Dq12Dq 11

33


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For transition between the same multiplicity states B is enough to explainFor transition between the same multiplicity states B is enough to explain

the positions of the bands.the positions of the bands.

For different multiplicity we need both B and C.For different multiplicity we need both B and C.

In dIn d33, for V, for V2+2+ ion, separation betweenion, separation between 44F andF and 22G is 4B + 3CG is 4B + 3C

B ~ 700B ~ 700--1000 cm1000 cm--11 and C is ~4Band C is ~4B


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Let us consider dLet us consider d33 (Cr(Cr3+3+) ion, B and C are known; B = 918cm) ion, B and C are known; B = 918cm--11, C = 4133 cm, C = 4133 cm--11

The possible transitions areThe possible transitions are

observedobserved predictedpredicted

44AA2g2g 44TT2g2g 11 = 14900= 14900 14900 (1014900 (10 DqDq))

44AA2g2g 44TT

1g1g(F)(F)

22= 22700= 22700 26800 (1826800 (18 DqDq))

44AA2g2g 44TT1g1g (P)(P) 33 = 34400= 34400 30700 (12Dq + 15B)30700 (12Dq + 15B)

Racah Parameter B for T.Racah Parameter B for T.

M. ions in cmM. ions in cm--11

MetalMetal MM2+2+ MM3+3+

TiTi 695695 --

VV 755755 861861

CrCr 810810 918918

MnMn 860860 965965with mixin

FeFe 917917 10151015

CoCo 971971 10651065

NiNi 10301030 11151115

Due to the mixing of P and F

terms, energy of 4T1g(P)

increased by an amount x

and that of 4T1g(F)

decreased by an amount x.

The splitting of the 4

P,4

F states in Cr3+

noo mixing4P

4F

15B

4T1g

(P)

4T1g (F)

4T2g

4A2g

6Dq 8Dq

10Dq12Dq 11

33

33


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B relates to a free ion, the apparent value of B in a complex is always lessB relates to a free ion, the apparent value of B in a complex is always less

than that of a free ion value because electron on the metal can be delocalizedthan that of a free ion value because electron on the metal can be delocalized

into MOs covering both metal and theinto MOs covering both metal and the ligandsligands

Use of B improves the agreement. This delocalization is calledUse of B improves the agreement. This delocalization is called nephelauxeticnephelauxetic

effect andeffect and nephelauxeticnephelauxetic ratio is given beratio is given be = B/B.= B/B.

decreases as delocalization increases, always,

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CH437Electronic Spectroscopy 1

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