Colour of metal complexes

Colour of metal complexes

1. Colour of metal
complexes

2. YouTubeLectures
•Thecolouroftransitionmetalcomplexes
–https://www.youtube.com/watch?v=B92pmPtg1uY
•Coloursoftransitionmetalcomplexes-ligandfieldeffect
–https://www.youtube.com/watch?v=J5pzOr9wXwU
•Whyaretransitionmetalcomplexescoloured?
–https://www.youtube.com/watch?v=8t8DUHYWNUc
&feature=emb_logo

3. •H
2
O and NH
3
are both colorless, but when they form a
coordinate compound they change color.
Blue green

4. Colour of transition metal complex ions
•Thestudyofthecolouroftransitionmetalcomplexions
canbemadeunderthefollowingtwoheads(A)and(B).
A)Colourofthecomplexionswhosecentralmetalatom
containspartially-filledd-orbitals
•Thetransitionmetalcomplexionsandcomplex
compoundswhosecentralatomcontainspartially-filledd-
orbitalsareusuallycolouredinthesolidformorin
solutionform.
•Whenwhitelightwhichiscomposedofmanydifferent
coloursfallsonacolouredcomplexionorcomplex
compoundofatransitionmetal,theionorthecompound
absorbssomeportionofthewhitelightandtheremaining
portionoftheincidentlightistransmittedorreflectedby
thecomplexcompoundorion.

5. Characteristicsofabsorbedlight
Theabsorbedlighthasthefollowingcharacteristics:
i)Coloursoftheabsorbedandtransmittedlight
•Sincewhitelightiscomposedofmanydifferentcolours,the
coloursoftheabsorbedlightandtransmittedlightaredifferent
fromeachother.
•Thecolourofthetransmittedlightiscalledcomplementary
colouroftheabsorbedlight(seefollowingtable).
•Asamatteroffact,thecolourofagivencomplexionor
compoundisthecolourofthetransmittedlight.
•Forexample:
a)SincehydratedCu
2+
ion,i.e.,Cu
2+
ionpresentin
[Cu(H
2
O)
5
]SO
4
orCuSO
4
.5H
2
Oabsorbsyellowradiation,it
transmitsblueradiationandhencethisionlooksbluetoour
eyes. [Cu(H
2
O)
5
]SO
4
or CuSO
4
.5H
2
O
Cu
2+
ion → Yellow radiation → Blue

6. Figure:The colour wheel shows the colour absorbed and their
complementary colours.

8. ← Transmitted light
↓
Complementary color

9. b)HydratedTi
3+
ion,i.e.,Ti
3+
ionpresentin[Ti(H
2
O)
6
]
3+
absorbs
greenradiationandhencetransmitstheradiationofpurple
colour.
•HydratedTi
3+
ion,therefore,looksviolet(almostpurple).
c)AnhydrousCo
2+
compoundsabsorbtheradiationofredcolour
andthereforeappearbluegreen.
•OntheotherhandthehydratedCo
2+
ion,i.e.,Co
2+
ionin
[Co(H
2
O)
6
]
2+
ion,absorbsbluegreenradiationandtherefore,
appearsred.
•Thecoloursofsomecommonhexahydratedtransitionmetal
complexions,[M(H
2
O)
6
]
n+
giveninfollowingtable.
[Ti(H
2
O)
6
]
3+
Ti
3+
ion → Green → Purple
Anhydrous Co
2+
Co
2+
ion → Red → Blue green
Hydrated Co
2+
Co
2+
ion → Blue green→ Red

10. Table:Coloursof some hexahydratedtransition metal complex
ions, [M(H
2
O)
6
]
n+
. For the sake of convenience, 6H
2
O molecules
have not been shown
(n = No. of unpaired electrons).

11. ii)Natureofthecentralmetalatom/ion
•Thecolouroftheabsorbedlightandhencethatofthecomplexion
orcompounddependsonthenatureofthecentralmetalatom/ion.
•ForexampleTi
3+
ionin[Ti(H
2
O)
6
]
3+
absorbsgreenradiationand
hence[Ti(H
2
O)
6
]
3+
ionlooksviolet(almostpurple).
•OntheotherhandCo
2+
ionin[Co(H
2
O)
6
]
2+
ionabsorbsbluegreen
radiationand,therefore,appearsred.
•ThusweseethatalthoughTi
3+
aridCo
2+
ionsareattachedwiththesame
ligands(viz.6H
2
Omolecules),theyabsorbtheradiationsofdifferent
colourandhence[Ti(H
2
O)
6
]
3+
and[Co(H
2
O)
6
]
2+
ionsappeardifferentin
theircolour.
iii)Natureoftheligands
•Thecolouroftheabsorbedlightalsodependsonthenatureoftheligands
•ForexampleNi
2+
ionin[Ni(NH
3
)
6
]
2+
ionabsorbsyellowradiationand
hencehasbluecolour.
[Ti(H
2
O)
6
]
3+
Ti
3+
ion → Green → Purple
[Co(H
2
O)
6
]
2+
Co
2+
ion → Blue green→ Red

12. •OntheotherhandNi
2+
ionin[Ni(H
2
O)
6
]
2+
ionabsorbsredcolourand
is,therefore,bluegreenincolour.
•Thus,weseethatalthoughthecentralmetalionisthesameinboththe
complexions,theseionsabsorbradiationsofdifferentcoloursand,
therefore,havedifferentcolours.
iv)Wavelengthoftheabsorbedlight
•Thewavelengthoftheabsorbedlightliesinthevisibleregionofthe
electromagneticradiation(4000-8000Å),i.e.,thetransitionmetal
ionsabsorbonlythatportionoftheincidentlightwhosewavelength
liesinbetween4000Åand8000Å(visibleregion).
v)Energycalculation
•Theenergyassociatedwiththewavelengthoftheradiationabsorbed
bythecomplexioncanalsobecalculated.
•Suppose[M(H
2
O)
6
]
n+
ionwhichisacomplexionabsorbstheradiation
whosewavelengthisequaltoλÅ.
[Ni(NH
3
)
6
]
2+
Ni
2+
ion → Yellow → Blue
[Ni(H
2
O)
6
]
2+
Ni
2+
ion → Red → Blue green

13. •Obviouslythefrequencyinwavenumber(incm
-1
)
correspondingtothiswavelengthisgivenby:
•Frequencyinwavenumber(incm
-1
)oftheabsorbed
radiationofwavelengthofλÅ:
•Thisfrequencyisassociatedwithenergywhose
magnitudecanbecalculatedinkcalmol
-1
orKJmol
-1
by
usingthefactthatafrequencyof350cm
-1
isassociated
withenergyequalto1kcalmol
-1
andafrequencyof83.7
cm
-1
isassociatedwithenergyequalto1kJmol
-1
,i.e.,
f = 1/λ

15. •Equation(ii)and(iv)bothshowthattheenergyassociated
withtheabsorbedradiationisinverselyproportionalto
thewavelengthoftheradiationbutdirectlyproportional
tothefrequencyoftheradiation.
Toexplainthepurplecolourofoctahedral[Ti(H
2
O)
6
]
3+
ionbyd-delectrontransition
•[Ti(H
2
O)
6
]
3+
isanoctahedralcomplexioninwhichTi-
atomispresentasTi
3+
ionwhosevalence-shellelectronic
configurationis3d
1
.
•Accordingtocrystalfieldtheorywhensixwatermolecules(which
actasligands)approachthecentralTi
3+
iontoformtheoctahedral
complexion,[Ti(H
2
O)
6
]
3+
,thefived-orbitalsofTi
3+
ionsplitinto
t
2g
(d
xy
,d
yz
andd
zx
)ande
g
(d
x
2
-y
2
andd
z
2
)setsoforbitals.
•Thisphenomenoniscalledcrystalfieldsplitting.
•t
2g
setoforbitalshaslowerenergythane
g
setoforbitalsandhence
d
1
electronofTi
3+
ionresidesint
2g
setande
g
setremainsvacant.

16. •ThusthegroundstateelectronicconfigurationofTi
3+
ion
canbewrittenast
1
2g
e
0
g
.
•Nowwhenwhitelightisallowedtofailon[Ti(H
2
O)
6
]
3+
ion,thisionabsorbsgreenradiation.
•Sincetheionabsorbsgreenradiation,thecolourofthe
transmittedradiationwouldbewhitelightminusgreen
light(colouroftheabsorbedlight)whichispurple.
•Thus[Ti(H
2
O)
6
]
3+
ionlookspurpletooureyes.
•Theabsorptionofgreenlightby[Ti(H
2
O)
6
]
3+
iontakes
placeatawavelengthofabout5000Åasisevidentfrom
thevisibleabsorptionspectrumwhichhasbeenobtained
byplottingagraphbetweenthewavelength(inÅ)ofthe
absorbedlightandtheamountofabsorbedlight
(absorbance)(seefollowingfigure).

18. •5000Åisthewavelengthofthebandofmaximum
absorption.
•Obviouslytheenergy(Δ
o
)associatedwiththewavelength
of5000Å.
•Thisenergy(=239kJmol
-1
)isclosetothecrystalfield
splittingenergy,Δ
o
(alsocalledexcitationenergy)
(Excitationenergyistheenergydifferencebetweent
2g
ande
g
orbitals)and,therefore,theelectronpresentint
2g
orbitalsabsorbs239kJmol
-1
andisexcitedtooccupythe
vacante
g
orbitals(t
1
2g
→e
0
g
electrontransition)sothat
theelectronicconfigurationofTi
3+
ionintheexcitedstate
becomest
0
2g
e
1
g
(seefollowingfigure).
•Thusweseethatthepurplecolourof[Ti(H
2
O)
6
]
3+
ionisduetothe
excitationofanelectronfromthed-orbitalsoflowerenergy(t
2g
orbitals)tothed-orbitalsofhigherenergy(e
g
orbitals).

20. •WehavealreadysaidthatthemagnitudeofΔ
o
(Δ
o
isthe
energydifferencebetweent
2g
ande
g
orbitals)isinversely
proportionaltothatofthewavelengthoftheabsorbed
radiation.
•ThismeansthatacomplexionhavinghighervalueofΔ
o
will
absorbtheradiationoflowerwavelengthandviceversa.
Evidences
•Thisfactisevidentfromthefollowingexamplesofoctahedral
complexionsofCo
3+
andNi
2+
ions:
a)SincethevalueofΔ
o
for[Co(CN)
6
]
3-
ionishigherthanthatof
[Co(NH
3
)
6
]
3+
(seefollowingtable).
•[Co(CN)
6
]
3-
ionabsorbsvioletradiationwhichhaslower
wavelength.
Higher Δ
o
→ lower λ
Higher Δ
o
for [Co(CN)
6
]
3-
→ lower Δ
o
for [Co(NH
3
)
6
]
3+
[Co(CN)
6
]
3-
→ violet → lower λ→ yellow green

21. •Ontheotherhand[Co(NH
3
)
6
]
3+
ionabsorbsbluegreenradiation
whichhashigherwavelength.
•Thus[Co(CN)
6
]
3-
ionappearsyellowgreeninsolutionand
[Co(NH
3
)
6
]
3+
ionappearsorangeorpinkinsolution.
b)Since[Ni(H
2
O)
6
]
2+
ionhaslowervalueofΔ
o
than[Ni(NH
3
)
6
]
2+
ion,
[Ni(NH
3
)
6
]
2+
ionabsorbsyellowcolourwhichhaslower
wavelengthwhile[Ni(H
2
O)
6
]
2+
ionabsorbsredcolourwhichhas
higherwavelength.
•Thus[Ni(NH
3
)
6
]
2+
ioninsolutionhasbluecolourbut[Ni(H
2
O)
6
]
2+
ioninsolutionhasbluegreencolour.
[Co(NH
3
)
6
]
3+
→ blue green → higher λ→ orange or pink
[Ni(NH
3
)
6
]
2+
Ni
2+
ion → Yellow → Blue
[Ni(H
2
O)
6
]
2+
Ni
2+
ion → Red → Blue green
[Ni(NH
3
)
6
]
2+
→ Yellow →λ5750 -5900Å→ Δ
o
10800 cm
-1
→ Blue
[Ni(H
2
O)
6
]
2+
→ Red →λ6250 -7500Å → Δ
o
8500 cm
-1
→ Blue green

22. (Orange or pink)

23. Table: Octahedral complexes of Cr
3+
and their colors.
Octahedral complexes Colors

24. B)Colourofthecomplexionswhosecentralatomcontainsempty
orcompletely-filledd-orbitals
•Thetransitionmetalcomplexionswhosecentralatomcontains
empty(d
0
configuration)orcompletely-filledd-orbitals(d
10
configuration)arecolourless.
•Thisisbecauseofthefactthatsincethed-orbitalsofthecentral
metaliondonotcontainanyelectron(emptyd-orbitals)orare
completely-filled(i.e.,containalltheelectronsinthepairedstate),
d-delectrontransitionisnotpossibleandhencenolightofany
colourorwavelengthisabsorbedbysuchionsinthevisibleregion.
•Thusthetransitionmetalionscontaining:
–Emptyd-orbitals[e.g.,Sc
3+
andTi
4+
(TiO
2
)ions)]or
–Completely-filledd-orbitals[e.g.,Cu
+
,Ag
+
,Zn
2+
(ZnSO
4
),Cd
2+
,
Hg
2+
etc.]
arecolourless.