Ligand Field Theory Fundamentals - Chemistry LibreTexts

Ligand Field Theory can be considered an extension of Crystal Field Theory such that all levels of covalent interactions can be incorporated ... Skiptomaincontent ContributorsandAttributions LigandFieldTheorycanbeconsideredanextensionofCrystalFieldTheorysuchthatalllevelsofcovalentinteractionscanbeincorporatedintothemodel.TreatmentofthebondinginLFTisgenerallydoneusingMolecularOrbitalTheory.Aqualitativeapproachthatcanbeusedforoctahedralmetalcomplexesisgiveninthefollowing3diagrams. Inthefirstdiagram,the3d,4sand4pmetalionatomicorbitalsareshowntogetherwiththeligandgrouporbitalsthatwouldhavethecorrectsymmetrytobeabletooverlapwiththem.Thesymmetryadaptedlinearcominationofligandorbitalsaregeneratedbytaking6sigmaorbitalsfromtheligands,designatedasσx,σ-x,σy,σ-y,σz,σ-zandthencombiningthemtomake6ligandgrouporbitals.(labelledeg,a1g,t1u) Intheseconddiagramonlysigmabondingisconsideredanditshowsthecombinationofthemetal3d,4sand4porbitalswithOCCUPIEDligandgrouporbitals(using1orbitalfromeachligand).Theresultisthatthatthemetalelectronswouldbefedintot2gandeg*molecularorbitalswhichissimilartotheCFTmodelexceptthattheegorbitalisnoweg*. Forexample:B-[M(II)I6]4-A-[M(II)(H2O)6]2+C-[M(II)(CN)6]4- Inthethirddiagram,π(pi)bondingisconsidered.Ingeneralπbondsareweakerthanσ(sigma)bondsandsotheeffectistomodifyratherthandramaticallyalterthedescription.2orbitalsfromeachligandarecombinedtogiveatotalof12whicharesubdividedintofoursetswiththreeligandgrouporbitalsineachset.Thesearelabelledt1g,t1u,t2gandt2u.Themetalt2gorbitalisthemostsuitableforinteractionandthisisshowninthe2casesabove. CaseAisthesameasabove,ignoringπinteractions. ForcaseB,theligandπorbitalsarefullandatlowerenergythanthemetalt2g.ThiscausesadecreaseinthesizeofΔ. ForcaseC,theligandπorbitalsareemptyandathigherenergythanthemetalt2g.ThiscausesanincreaseinthesizeofΔ. ReturningtotheproblemofcorrectlyplacingligandsintheSpectrochemicalseries,thehalidesareexamplesofcaseAandgroupslikeCN-andCOareexamplesofcaseB.ItispossiblethentoexplaintheSpectrochemicalseriesoncecovalenteffectsareconsidered. SomeconvincingargumentsforcovalencyandeffectsonΔcomefromastudyoftheIRspectrarecordedforsimplecarbonylcompoundse.g.M(CO)6. TheCOmoleculehasastrongtriplebondwhichintheIRgivesrisetoastrongabsorptionat~2140cm-1.Fortheseries[Mn(CO)6]+,[Cr(CO)6]and[V(CO)6]-,whichareisoelectronic,theIRbandsfortheCOhaveshiftedto2090,2000and1860cm-1respectively.Despitethefactthatthemetalshavethesamenumberofelectrons(isoelectronic)thefrequencyofforceconstantoftheCObondisseentovaryMn+>Cr>V-. Thiscannotbeexplainedonanionicbasisbutisconsistentwiththeπbondingschemesincethegreaterthepositivechargeonthemetal,thelessreadilythemetalcandelocalizeelectronsbackintotheπ*orbitalsoftheCOgroup. NotethattheIRvalueswearedealingwithrelatetotheCObondandnottheM-CsowhentheCOfrequencygetslessthenitislosingtriplebondcharacterandbecomingmorelikeadoublebond.Thisisexpectedifelectronsarepushedbackfromthemetalintowhatwereemptyπ*antibondingorbitals. ContributorsandAttributions Prof.RobertJ.Lancashire(TheDepartmentofChemistry,UniversityoftheWestIndies)