Ligand Field Transitions - Every Science

The LFSE therefore depends on the number of electrons in the d-orbitals of the metal, x+y, the value of Δo, and the distribution of electrons between the ... Skiptocontent Whenametalissubjectedtotheperturbationofanoctahedralfield,theenergiesofthed-orbitalssplitintotwogroups,thelower energy t2g,at-0.4Δo,andthehigherenergyeg,at0.6Δo,whereΔo isthe ligand fieldsplittingparameter. Whenelectronsareputintotheseorbitals,theorbitalswhichbecomeoccupieddependonthevalueofΔo.Whenthereare x electronsinthet2g orbitals,andyelectronsintheeg orbitals,thetotalenergyoftheelectronsrelativetotheaverageenergyoftheelectronsisknownasthe LigandFieldStabilizationEnergy (LFSE): LFSE=(-0.4x+0.6y)ΔDo TheLFSEthereforedependsonthenumberofelectronsinthed-orbitalsofthemetal,x+y,thevalueofΔo,andthedistributionofelectronsbetweenthet2g andeg levels. Whencalculatingtheelectronicconfigurationofatransitionmetal complex,weshouldnotethattheelectronsalwaysoccupythelowestenergyconfiguration. ElectronicConfigurationsofd-metalcomplexes d1: t2g1:LFSE=-0.4Δo d2: t2g2:LFSE=-0.8Δo d3: t2g3:LFSE=-1.2Δo d4: t2g4:LFSE=-1.6Δo d4: t2g3eg1:LFSE=-0.6Δo Aswecansee,whenthenumberofd-electronsis0,1,2,or3,thereisnotroubleassigninganelectronicconfiguration,itbeingt2gn.However,whena4thelectronisadded,therearetwopossibleconfigurations:t2g4 andt2g3eg1.ThesetwoconfigurationsdifferinLFSEbyΔo,andsoitwouldbepredictedthatthet2g4 configurationwouldbelowerinenergy.However,thisconfigurationinvolvesthepairingofelectronsinoneofthet2g orbitals,andtheextrarepulsionassociatedwithpairedelectrons,withenergyP,inthesame orbital actstodestabilizethet2g4 configuration. Weneedtoconsidertheoverallstabilizationenergy,SE,whichistheligandfieldstabilizationenergy,LFSE,plusthepairingenergy,PE. Overallstabilizationenergyofad4 complex t2g4: SE=LFSE+PE =-1.6Δo +P t2g3eg1: SE=LFSE+PE =-0.6Δo +0                           =-1.6Δo +Δo Theconfigurationadoptedthereforedependsupontherelativemagnitudeofthesplittingparameter,Δo,andthepairingenergy,P.If Δo

P,thelowert2g orbitalisoccupiedtomaximizetheLFSE.Pdoesnotchange,foragivenelement,andsotheconfigurationisdeterminedbythevalueofΔo.Thefirstsituation,withconfigurationt2g3eg1 isknownasthe weak-field limit,andthesecond,withconfigurationt2g4,isknownasthe strong-field limit. IfweconsidertheMOdiagramsforthetwod4 complexes,weseethatintheweak-fieldlimit,alltheelectronspinsareparallel,andtheoverallelectronspinis2Inthestrong-fieldlimit,twooftheelectronsarepaired,andhencehaveantiparallelspins,sotheoverallelectronspinis1.Whenthereisachoiceofpossibleelectronicconfigurations,theconfigurationwiththelowestnumberofparallelelectronspinsisknownasthe low-spinconfiguration,anditcorrespondstothestrongfield,andtheconfigurationwiththehighestnumberofparallelelectronspinsisknownatthe high-spinconfiguration,anditcorrespondstotheweak-fieldlimit. Similarargumentscanbeconstructedford5,d6,andd7 complexes,butford8,d9,andd10,thereisagainonlyonepossibleconfiguration. Ligandfieldtransitions occurwhenanelectronisexcitedfromanorbitalwithoneenergytoanorbitalwithanotherenergy.Oneexampleisthet2g-to-eg transitionfromwhichtheLFSE,Δo,maybecalculated.Thesewillsometimesinvolveachangeintheelectronspin,andhencehaveaneffectonthemagneticpropertiesifthecomplex:the magneticproperties ofthecomplexaredeterminedbythenumberofunpairedelectrons. Searchfor: PopularPosts