Crystal Field Theory - Chemistry LibreTexts

Crystal field theory (CFT) is a bonding model that explains many important properties of transition-metal complexes, including their colors, ... Skiptomaincontent Noheaders Oneofthemoststrikingcharacteristicsoftransition-metalcomplexesisthewiderangeofcolorstheyexhibit.Crystalfieldtheory(CFT)isabondingmodelthatexplainsmanyimportantpropertiesoftransition-metalcomplexes,includingtheircolors,magnetism,structures,stability,andreactivity.ThecentralassumptionofCFTisthatmetal–ligandinteractionsarepurelyelectrostaticinnature. ColorsofCoordinationComplexesThecolorforacoordinationcomplexcanbepredictedusingtheCrystalFieldTheory(CFT).Knowingthecolorcanhaveanumberofusefulapplications,suchasthecreationofpigmentsfordyesinthetextileindustry.Thetendencyforcoordinationcomplexestodisplaysuchawidearrayofcolorsismerelycoincidental;theirabsorptionenergieshappentofallwithinrangeofthevisiblelightspectrum.CrystalFieldStabilizationEnergyAconsequenceofCrystalFieldTheoryisthatthedistributionofelectronsinthedorbitalsmayleadtonetstabilization(decreaseinenergy)ofsomecomplexesdependingonthespecificligandfieldgeometryandmetald-electronconfigurations.Itisasimplemattertocalculatethisstabilizationsinceallthatisneededistheelectronconfigurationandknowledgeofthesplittingpatterns.CrystalFieldTheoryCrystalfieldtheory(CFT)describesthebreakingoforbitaldegeneracyintransitionmetalcomplexesduetothepresenceofligands.CFTqualitativelydescribesthestrengthofthemetal-ligandbonds.Basedonthestrengthofthemetal-ligandbonds,theenergyofthesystemisaltered.Thismayleadtoachangeinmagneticpropertiesaswellascolor.ThistheorywasdevelopedbyHansBetheandJohnHasbrouckvanVleck.IntroductiontoCrystalFieldTheoryOneofthemoststrikingcharacteristicsoftransition-metalcomplexesisthewiderangeofcolorstheyexhibit.Inthissection,wedescribecrystalfieldtheory(CFT),abondingmodelthatexplainsmanyimportantpropertiesoftransition-metalcomplexes,includingtheircolors,magnetism,structures,stability,andreactivity.ThecentralassumptionofCFTisthatmetal–ligandinteractionsarepurelyelectrostaticinnature.MagneticMomentsofTransitionMetalsMagneticmomentsareoftenusedinconjunctionwithelectronicspectratogaininformationabouttheoxidationnumberandstereochemistryofthecentralmetalionincoordinationcomplexes.AcommonlaboratoryprocedureforthedeterminationofthemagneticmomentforacomplexistheGouymethodwhichinvolvesweighingasampleofthecomplexinthepresenceandabsenceofamagneticfieldandobservingthedifferenceinweight.Atemplateisprovidedforthecalculationsinvolved.MagnetismMovementofanelectricalcharge(whichisthebasisofelectriccurrents)generatesamagneticfieldinamaterial.Magnetismisthereforeacharacteristicpropertyofallmaterialsthatcontainelectricallychargedparticlesandformostpurposescanbeconsideredtobeentirelyofelectronicorigin.Metals,TetrahedralandOctahedralNon-octahedralComplexesOctahedralvs.TetrahedralGeometriesAconsequenceofCrystalFieldTheoryisthatthedistributionofelectronsinthedorbitalscanleadtostabilizationforsomeelectronconfigurations.Itisasimplemattertocalculatethisstabilizationsinceallthatisneededistheelectronconfiguration.OrgelDiagramsOrgeldiagramsareusefulforshowingtheenergylevelsofbothhighspinoctahedralandtetrahedraltransitionmetalions.QualitativeOrgelDiagramsTanabe-SuganoDiagramsTanabeSuganodiagramsareusedtopredictthetransitionenergiesforbothspin-allowedandspin-forbiddentransitions,aswellasforbothstrongfield(lowspin),andweakfield(highspin)complexes.Inthismethodtheenergyoftheelectronicstatesaregivenontheverticalaxisandtheligandfieldstrengthincreasesonthehorizontalaxisfromlefttoright.Tetrahedralvs.SquarePlanarComplexesHighspinandlowspinaretwopossibleclassificationsofspinstatesthatoccurincoordinationcompounds.Theseclassificationscomefromeithertheligandfieldtheory,whichaccountsfortheenergydifferencesbetweentheorbitalsforeachrespectivegeometry,orthecrystalfieldtheory,whichaccountsforthebreakingofdegenerateorbitalstates,comparedtothepairingenergy.ThermodynamicsandStructuralConsequencesofd-OrbitalSplittingTheenergylevelsplittingofthed-orbitalsduetotheirinteractionwiththeligandsinacomplexhasimportantstructuralandthermodynamiceffectsonthechemistryoftransition-metalcomplexes.Althoughthesetwotypesofeffectsareinterrelated,theyareconsideredseparatelyhere.