Chelates and chelate effect

Chelates and chelate effect

1. Chelatesand chelate
effect

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

3. Ligands
•Theneutralmoleculesorions(usuallyanions)whichareattached
withthecentralmetalionarecalledligands. OR
Incoordinationchemistry,aligand is
anionormolecule(functionalgroup)thatbindstoa
centralmetalatomtoformacoordinationcomplex.
Thebondingbetweenmetalandligandgenerallyinvolvesformal
donationofoneormoreoftheligand'selectronpairs.
Thenatureofmetal-ligandbondingcanrangefromcovalentto
ionic.
Furthermore,themetal-ligandbondordercanrangefromoneto
three.
TypesofLigands
–Neutralligands(NH
3
,CO)
–Positiveligands(NH
2
-NH
3
+
)
–Negativeligands(CN
-
,CNS
-
,OH
-
)

4. •Forexample,inthecomplexion,[Fe(CN)
6
]
3-
thesix(CN
-
)ions
aretheligands.
•Inmostofcomplexesaligandactsasadonorpartner,i.e.,donates
one(ormore)electronpairtothecentralmetal.
•NotethatinmetalliccarbonylstheligandvizCOmoleculeacts
bothasdonorandacceptor(MCO).
Substrate
•Asubstratemaybedefinedasthereactantinwhichsomebonds
arebrokenandsomenewonesareformedasaresultoftheattack
ofareagent.
Substrate
Product
Attacking reagent Attacking reagent

5. Chelates
•Acompoundcontainingaligand(typicallyorganic)
bondedtoacentralmetalatomattwoormorepoints.
•Achelatingligandisabidendateorpolydentateligand
whichisattachedtothesamecentralmetalatombytwo
ormoreofitsdonoratomsresultingintheformationofa
complexhavingastrain-freeringstructure.
•Thecomplexhavingtheringstructureiscalledchelateor
chelatedcomplex.
•Thechelateisalsocalledbyvariousothernameslike
cycliccomplex,ring-typecomplexetc.
•Theformationofachelateiscalled chelationor
cyclisation.

6. •Thetermchelatewasfirstappliedin1920byGT
MorganandHDKDrew.
•Theadjective(تفص)chelate,derivedfromthegreatclaw
(ہجنپ)orchela( ےڑکيک ۔ وهچب )(fromtheGreek)ofthe
lobster(اگنيهچ)orothercrustaceans( لوخ راد),is
suggestedforthecaliper( رادمخ اي یکنهد راکرپ )likegroups
whichfunctionastwoassociatingunitsandfastentothe
centralatomsoastoproduceheterocyclicrings.

13. Chelateeffect(Stabilityofchelates)
•Chelatesaremorestablethansimilarnon-chelatedcomplexesofthesame
metalion.
•Greaterstabilityofchelatedcomplexmeansthattherewillbesmaller
dissociationofthechelatedcomplexintoitscomponentsinaparticular
solvent.
•Theenhancedstabilityconferredonachelateiscalledchelateeffect.
•Thegreaterstabilityofachelatedcomplexascomparedtothatofanon-
chelatedcomplexisduetotheincreaseinentropy.
•Thuswhenweconsidertheformationof[Co(NH
3
)
6
]
2+
(non-chelated
complex)and[Co(en)
3
]
2+
(chelatedcomplex)from[Co(H
2
O)
6
]
2+
as
representedbyreactions(a)and(b)givenbelow:

14. •Wefindthatnumberofparticlesontherighthandsideofreaction
(b)whichrepresentstheformationofchelatedcomplexisgreater
thanthatonthelefthandsidewhilethenumberofparticlesonboth
sidesofreaction(a)representingtheformationofnon-chelated
complexisthesame.
•Consequentlythechelatedcomplex,[Co(en)
3
]
2+
ismorestablethan
thenon-chelatedcomplex,[Co(NH
3
)
6
]
2+
.
•Thestabilityofchelatedcomplexesincreaseswiththeincreaseof
thenumberofdonoratomspresentinthechelatingligand.
•Thestabilityofchelatedcomplexesalsodependsonthesizeofthe
ringformedinthecomplex.
•Thechelatedcomplexescontaining4-memberedringsarenotstable
whilethosecontaining5-and6-memberedringarequitestable.
•Achelatedcomplexgivenbyapolydentateligandcontaining
doublebondsismorestablethanthatgivenbyapolydentateligand
containingonlysinglebonds.

15. •Forexampleanoctahedralcomplexofacetylacetonatoligand,
ismorestablethantheoctahedralcomplexofethylenediaminewith
thesamemetalion,[M
2+
(NH
2
—CH
2
—CH
2
—NH
2
)
3
]
2+
.
•Notethatboththecomplexesarechelatedcomplexes.
•Withtheincreaseofthenumberoftheringspresentinthechelated
complex,thecomplexbecomesmorestable.

24. The End