Spectrochemical series

Spectrochemical series

1. Spectral properties
•Sana 524
YouTubeVideo
•SpectrochemicalSeries
•https://www.youtube.com/watch?v=CedCBlRK0Bs
•UsingtheSpectrochemicalSeriestodrawametalcomplexion's
crystalfieldsplitting
•https://www.youtube.com/watch?v=h47CZz-t9iE

2. Course Outlines
Chemistryofd-BlockElementsandCoordinationComplexes
•Backgroundofcoordinationchemistry→Done
•Generalchemicalandphysicalpropertiesoftransitionelements→
Done
•Comparisonoftheelementsoffirsttransitionseries(3d)with
thoseofsecond(4d)andthird(5d)series→Done
•NomenclatureandStructureofcoordinationcomplexeswith
coordinationnumber2-6→Done
•Chelatesandchelateeffect→Done
•Theoriesofcoordinationcomplexes
•Werner'sTheory→Done
•ValenceBondTheory(VBT)
•CrystalFieldTheory(CFT)
•MolecularOrbitalTheory(MOT)
•Sidgwick’selectronicinterpretation
ofcoordination→Done
•Effectiveatomicnumber(EAN)
•Jahn-Tellertheorem
•Magneticproperties→Done
•Spectralproperties →
Continue
•Isomerism
•Stereochemistry
•Stabilityconstantsof
coordinationcomplexes

3. Spectrochemical
series
A spectrochemicalseries is a list of ligands ordered on ligand strength
and a list of metal ions based on oxidation number, group and its
identity.
The spectrochemicalseries is a list of ligands based on the strength of
their interaction with metal ions
The spectrochemicalseries ranks ligands according the energy difference
Δ (Δ
o
) between the t
2g
and e
g
orbitals in their (octahedral) complexes.

4. Spectrochemical series
•Aspectrochemicalseriesisalistofligandsorderedon
ligandstrengthandalistofmetalionsbasedonoxidation
number,groupanditsidentity.
•Incrystalfieldtheory,ligandsmodifythedifferencein
energybetweenthedorbitals(Δ)calledtheligand-field
splittingparameterforligandsorthecrystal-fieldsplitting
parameter,whichismainlyreflectedindifferencesin
colorofsimilarmetal-ligandcomplexes.
Spectrochemicalseriesofligands
•ThemagnitudeofΔ
o
variesfromstrong(er)toweak(er)ligands.
•Strong(er)ligandsarethosewhichexertastrong(er)fieldonthe
centralmetalionandhencehavehighersplittingpowerwhilethe
weak(er)ligandsarethosewhichhaveaweak(er)fieldonthe
centralmetalcationandconsequentlyrelativelylowersplitting
power.

5. •Thusstrong(er)ligands(e.g.CN
-
)givelargervalueofΔ
o
and
weak(er)ligands(e.g.F
-
)yieldasmallervalueofΔ
o
(seefollowing
figure).
Figure: Splitting of five d-orbitals in presence of strong(er) and weak(er)
ligandsin an octahecralcomplex. (a) Five d-orbitals in the free metal ion (b)
Splitting of d-orbitals in presence of strong(er) ligands(c) Splitting of d-
orbitals in presence of weak(er) ligands.

6. •AbovefigureshowsthatnotonlyΔ
o
,whichrepresentsthe
energydifferencebetweenthet
2g
ande
g
-setsoforbitals,is
smallerintheweak(er)fieldcomplexthaninthe
strong(er)field,butalsothatboththet
2g
ande
g
-levelsof
theweak(er)fieldarecorrespondinglyclosertothelevel
ofthedegeneratefived-orbitalsofthefreeisolated
metallicionthanarethose,respectively,ofthestrong(er)
field.
•Thecommonligandscanbearrangedintheorderoftheir
increasingsplittingpowertocaused-orbitalssplitting.
•Thisseriesiscalledspectrochemicalseries.
•Foroctahedralcomplexes,∆
oct
increasesalongthefollowing
spectrochemicalseriesofligands;the[NCS]
-
ionmaycoordinate
throughtheN-orS-donor(SCNmeanstheligandisboundvia
sulfurandNCSvianitrogen)andaccordingly,ithastwopositions
intheseries:

7. •Thespectrochemicalseriesisreasonablygeneral.
•Ligandswiththesamedonoratomsareclosetogetherintheseries.
•ThisseriesshowsthatthevalueofΔ
o
intheseriesalsoincreasesfromleft
toright.
•TheincreaseinthevalueofΔ
o
onproceedingfromlefttorightinthe
spectrochemicalseriesisquiteevidentfromthevaluesofΔ
o
forsome
octahedralcomplexesgiveninfollowingtablewhichclearlyshowsthat
sinceonproceedingfrom6Br
-
→3en,thefieldstrengthoftheligands
increases,thevalueofΔ
o
alsocorrespondinglyincreases.
Small Δ Large Δ
Weak-field ligands Strong-field ligands
π-Donor ligands π-Acceptor ligands
High-spincomplexes Low-spincomplexes

8. Characteristicsofoctahedralcomplexes
•Ifweconsideroctahedralcomplexesofd-blockmetalions,a
numberofpointsarisewhichcanbeillustratedbythefollowing
examples:
Table:Δ
o
values (i.e., energy difference between t
2g
and e
g
levels) in
cm
-1
for some octahedral complexes.

9. –ThecomplexesofCr(III)listedinfollowingtableillustratethe
effectsofdifferentligandfieldstrengthsforagivenM
n+
ion;
–ThecomplexesofFe(II)andFe(III)infollowingtableillustrate
thatforagivenligandandagivenmetal,∆
oct
increaseswith
increasingoxidationstate;
Table:Valuesof ∆
oct
for some d-block metal complexes.

10. –WhereanalogouscomplexesexistforaseriesofM
n+
metalsions(constantn)inatriad,∆
oct
increases
significantlydownthetriad(e.g.followingfigure):
–Foragivenligandandagivenoxidationstate,∆
oct
variesirregularlyacrossthefirstrowofthed-block,e.g.
overtherange8000to14000cm
-1
forthe[M(H
2
O)
6
]
2+
ions.
Figure.Thetrendinvalues
of∆
oct
forthecomplexes
[M(NH
3
)
6
]
3+
whereM=Co,
Rh,Ir.
[Co(III) < Rh(III) < Ir(III)]

11. Spectrochemicalseriesofmetals
•Trendsinvaluesof∆
oct
leadtotheconclusionthatmetal
ionscanbeplacedinaspectrochemicalserieswhichis
independentoftheligands:
•Spectrochemicalseriesareempiricalgeneralizationsandsimple
crystalfieldtheorycannotaccountforthemagnitudesof∆
oct
values.
•Ingeneral,itisnotpossibletosaywhetheragivenligandwillexert
astrongfieldoraweakfieldonagivenmetalion.
•However,whenweconsiderthemetalion,thefollowingtwouseful
trendsareobserved:
–∆increaseswithincreasingoxidationnumber
–∆increasesdownagroup

12. Example
•Whichcomplexofthefollowingpairshasthelarger
valueofΔ
0
:
–[Co
III
(CN)
6
]
3-
and[Co
III
(NH
3
)
6
]
3+
–[Co
III
(NH
3
)
6
]
3+
and[Co
III
F
6
]
3-
–[Co
III
(H
2
O)
6
]
3+
and[Rh
III
(H
2
O)
6
]
3+
–[Co(H
2
O)
6
]
2+
and[Co
III
(H
2
O)
6
]
3+
Solution
[Co
III
(CN)
6
]
3-
and[Co
III
(NH
3
)
6
]
3+
•SinceCN
-
isstrongerthanNH
3
(i.e.CN
-
ionhasgreater
fieldstrengththanNH
3
),Δ
0
for[Co
III
(CN)
6
]
3-
isgreaterthan
Δ
0
for[Co
III
(NH
3
)
6
]
3+
.
[Co
III
(NH
3
)
6
]
3+
and[Co
III
F
6
]
3-
•[Co
III
(NH
3
)
6
]
3+
ionhasgratervalueofΔ
0
thanthatfor
[Co
III
F
6
]
3-
.

13. [Co
III
(H
2
O)
6
]
3+
and[Rh
III
(H
2
O)
6
]
3+•Rhisamemberof4d-serieswhileCoisamemberof3d-
series(3d→4d).
•Thusprincipalquantumnumber,n,forRhisgreaterthan
thatofCoandconsequentlyΔ
0
for[Rh
III
(H
2
O)
6
]
3+
is
greater.
[Co(III) < Rh(III) < Ir(III)]
[Co(H
2
O)
6
]
2+
and[Co
III
(H
2
O)
6
]
3+
•ThechargeonCoin[Co
III
(H
2
O)
6
]
3+
ishigherthanthaton
Co[Co(H
2
O)
6
]
2+
.
•ThusΔ
0
for[Co
III
(H
2
O)
6
]
3+
ishigher.