Ligand field theory - Wikipedia

Ligand field theory (LFT) describes the bonding, orbital arrangement, and other characteristics of coordination complexes. ... It represents an application of ... Ligandfieldtheory FromWikipedia,thefreeencyclopedia Jumptonavigation Jumptosearch Molecularorbitaltheoryappliedtotransitionmetalcomplexes Ligandfieldtheory(LFT)describesthebonding,orbitalarrangement,andothercharacteristicsofcoordinationcomplexes.[1][2][3]Itrepresentsanapplicationofmolecularorbitaltheorytotransitionmetalcomplexes.Atransitionmetalionhasninevalenceatomicorbitals-consistingoffivend,one(n+1)s,andthree(n+1)porbitals.Theseorbitalsareofappropriateenergytoformbondinginteractionwithligands.TheLFTanalysisishighlydependentonthegeometryofthecomplex,butmostexplanationsbeginbydescribingoctahedralcomplexes,wheresixligandscoordinatetothemetal.Othercomplexescanbedescribedbyreferencetocrystalfieldtheory.[4] Contents 1History 2Bonding 2.1σ-bonding(sigmabonding) 2.2π-bonding(pibonding) 3Highandlowspinandthespectrochemicalseries 4Seealso 5References 6Externallinks History[edit] Ligandfieldtheoryresultedfromcombiningtheprincipleslaidoutinmolecularorbitaltheoryandcrystalfieldtheory,whichdescribesthelossofdegeneracyofmetaldorbitalsintransitionmetalcomplexes.JohnStanleyGriffithandLeslieOrgel[5]championedligandfieldtheoryasamoreaccuratedescriptionofsuchcomplexes,althoughthetheoryoriginatedinthe1930swiththeworkonmagnetismofJohnHasbrouckVanVleck.GriffithandOrgelusedtheelectrostaticprinciplesestablishedincrystalfieldtheorytodescribetransitionmetalionsinsolutionandusedmolecularorbitaltheorytoexplainthedifferencesinmetal-ligandinteractions,therebyexplainingsuchobservationsascrystalfieldstabilizationandvisiblespectraoftransitionmetalcomplexes.Intheirpaper,theyproposedthatthechiefcauseofcolordifferencesintransitionmetalcomplexesinsolutionistheincompletedorbitalsubshells.[5]Thatis,theunoccupieddorbitalsoftransitionmetalsparticipateinbonding,whichinfluencesthecolorstheyabsorbinsolution.Inligandfieldtheory,thevariousdorbitalsareaffecteddifferentlywhensurroundedbyafieldofneighboringligandsandareraisedorloweredinenergybasedonthestrengthoftheirinteractionwiththeligands.[5] Bonding[edit] Thissectiondoesnotciteanysources.Pleasehelpimprovethissectionbyaddingcitationstoreliablesources.Unsourcedmaterialmaybechallengedandremoved.(January2021)(Learnhowandwhentoremovethistemplatemessage) σ-bonding(sigmabonding)[edit] Inanoctahedralcomplex,themolecularorbitalscreatedbycoordinationcanbeseenasresultingfromthedonationoftwoelectronsbyeachofsixσ-donorligandstothed-orbitalsonthemetal.Inoctahedralcomplexes,ligandsapproachalongthex-,y-andz-axes,sotheirσ-symmetryorbitalsformbondingandanti-bondingcombinationswiththedz2anddx2−y2orbitals.Thedxy,dxzanddyzorbitalsremainnon-bondingorbitals.Someweakbonding(andanti-bonding)interactionswiththesandporbitalsofthemetalalsooccur,tomakeatotalof6bonding(and6anti-bonding)molecularorbitals Ligand-Fieldschemesummarizingσ-bondingintheoctahedralcomplex[Ti(H2O)6]3+. Inmolecularsymmetryterms,thesixlone-pairorbitalsfromtheligands(onefromeachligand)formsixsymmetryadaptedlinearcombinations(SALCs)oforbitals,alsosometimescalledligandgrouporbitals(LGOs).Theirreduciblerepresentationsthatthesespanarea1g,t1uandeg.Themetalalsohassixvalenceorbitalsthatspantheseirreduciblerepresentations-thesorbitalislabeleda1g,asetofthreep-orbitalsislabeledt1u,andthedz2anddx2−y2orbitalsarelabeledeg.Thesixσ-bondingmolecularorbitalsresultfromthecombinationsofligandSALCswithmetalorbitalsofthesamesymmetry. π-bonding(pibonding)[edit] πbondinginoctahedralcomplexesoccursintwoways:viaanyligandp-orbitalsthatarenotbeingusedinσbonding,andviaanyπorπ*molecularorbitalspresentontheligand. Intheusualanalysis,thep-orbitalsofthemetalareusedforσbonding(andhavethewrongsymmetrytooverlapwiththeligandporπorπ*orbitalsanyway),sotheπinteractionstakeplacewiththeappropriatemetald-orbitals,i.e.dxy,dxzanddyz.Thesearetheorbitalsthatarenon-bondingwhenonlyσbondingtakesplace. Exampleofπbackbondingwithcarbonyl(CO)ligands. Oneimportantπbondingincoordinationcomplexesismetal-to-ligandπbonding,alsocalledπbackbonding.ItoccurswhentheLUMOs(lowestunoccupiedmolecularorbitals)oftheligandareanti-bondingπ*orbitals.Theseorbitalsarecloseinenergytothedxy,dxzanddyzorbitals,withwhichtheycombinetoformbondingorbitals(i.e.orbitalsoflowerenergythantheaforementionedsetofd-orbitals).Thecorrespondinganti-bondingorbitalsarehigherinenergythantheanti-bondingorbitalsfromσbondingso,afterthenewπbondingorbitalsarefilledwithelectronsfromthemetald-orbitals,ΔOhasincreasedandthebondbetweentheligandandthemetalstrengthens.Theligandsendupwithelectronsintheirπ*molecularorbital,sothecorrespondingπbondwithintheligandweakens. Theotherformofcoordinationπbondingisligand-to-metalbonding.Thissituationariseswhentheπ-symmetryporπorbitalsontheligandsarefilled.Theycombinewiththedxy,dxzanddyzorbitalsonthemetalanddonateelectronstotheresultingπ-symmetrybondingorbitalbetweenthemandthemetal.Themetal-ligandbondissomewhatstrengthenedbythisinteraction,butthecomplementaryanti-bondingmolecularorbitalfromligand-to-metalbondingisnothigherinenergythantheanti-bondingmolecularorbitalfromtheσbonding.Itisfilledwithelectronsfromthemetald-orbitals,however,becomingtheHOMO(highestoccupiedmolecularorbital)ofthecomplex.Forthatreason,ΔOdecreaseswhenligand-to-metalbondingoccurs. Thegreaterstabilizationthatresultsfrommetal-to-ligandbondingiscausedbythedonationofnegativechargeawayfromthemetalion,towardstheligands.Thisallowsthemetaltoaccepttheσbondsmoreeasily.Thecombinationofligand-to-metalσ-bondingandmetal-to-ligand π-bondingisasynergiceffect,aseachenhancestheother. Aseachofthesixligandshastwoorbitalsofπ-symmetry,therearetwelveintotal.Thesymmetryadaptedlinearcombinationsofthesefallintofourtriplydegenerateirreduciblerepresentations,oneofwhichisoft2gsymmetry.Thedxy,dxzanddyzorbitalsonthemetalalsohavethissymmetry,andsotheπ-bondsformedbetweenacentralmetalandsixligandsalsohaveit(astheseπ-bondsarejustformedbytheoverlapoftwosetsoforbitalswitht2gsymmetry.) Highandlowspinandthespectrochemicalseries[edit] Mainarticle:Spinstates(delectrons) Seealso:Magnetochemistry Thissectiondoesnotciteanysources.Pleasehelpimprovethissectionbyaddingcitationstoreliablesources.Unsourcedmaterialmaybechallengedandremoved.(January2021)(Learnhowandwhentoremovethistemplatemessage) Thesixbondingmolecularorbitalsthatareformedare"filled"withtheelectronsfromtheligands,andelectronsfromthed-orbitalsofthemetalionoccupythenon-bondingand,insomecases,anti-bondingMOs.TheenergydifferencebetweenthelattertwotypesofMOsiscalledΔO(Ostandsforoctahedral)andisdeterminedbythenatureoftheπ-interactionbetweentheligandorbitalswiththed-orbitalsonthecentralatom.Asdescribedabove,π-donorligandsleadtoasmallΔOandarecalledweak-orlow-fieldligands,whereasπ-acceptorligandsleadtoalargevalueofΔOandarecalledstrong-orhigh-fieldligands.Ligandsthatareneitherπ-donornorπ-acceptorgiveavalueofΔOsomewherein-between. ThesizeofΔOdeterminestheelectronicstructureofthed4-d7ions.Incomplexesofmetalswiththesed-electronconfigurations,thenon-bondingandanti-bondingmolecularorbitalscanbefilledintwoways:oneinwhichasmanyelectronsaspossibleareputinthenon-bondingorbitalsbeforefillingtheanti-bondingorbitals,andoneinwhichasmanyunpairedelectronsaspossibleareputin.Theformercaseiscalledlow-spin,whilethelatteriscalledhigh-spin.AsmallΔOcanbeovercomebytheenergeticgainfromnotpairingtheelectrons,leadingtohigh-spin.WhenΔOislarge,however,thespin-pairingenergybecomesnegligiblebycomparisonandalow-spinstatearises. Thespectrochemicalseriesisanempirically-derivedlistofligandsorderedbythesizeofthesplittingΔthattheyproduce.Itcanbeseenthatthelow-fieldligandsareallπ-donors(suchasI−),thehighfieldligandsareπ-acceptors(suchasCN−andCO),andligandssuchasH2OandNH3,whichareneither,areinthemiddle. I−