Ligand-Field Theory

Ligand-field theory enables the 3d, 4s, and 4p orbitals on the metal to overlap with orbitals on the ligand to form the octahedral covalent bond skeleton that ... Ligand-FieldTheory NahklehGroup RobinsonGroup WeaverGroup BodnerGroup TransitionMetals TransitonMetals CoordinationComplexesandLigands NomenclatureofComplexes Isomers Bonding ValenceBondTheory CrystalFieldTheory LigandFieldTheory Ligand-FieldTheory Thevalence-bondmodelandthecrystalfieldtheoryexplainsomeaspectsofthechemistryofthetransitionmetals,butneither modelisgoodatpredictingallofthepropertiesoftransition-metalcomplexes. Athirdmodel,basedonmolecularorbitaltheory,wasthereforedeveloped thatisknownasligand-fieldtheory.Ligand-fieldtheoryismorepowerfulthaneitherthevalence-bondorcrystal-field theories.Unfortunatelyitisalsomoreabstract. Theligand-fieldmodelforanoctahedraltransition-metalcomplexsuchas theCo(NH3)63+ionassumesthatthe3d,4s,and4porbitalsonthemetaloverlapwithoneorbitaloneachofthesixligands toformatotalof15molecularorbitals,asshowninthefiguebelow. Sixoftheseorbitalsarebondingmolecularorbitals,whoseenergiesaremuchlowerthanthoseoftheoriginalatomicorbitals. Anothersixareantibondingmolecularorbitals,whoseenergiesarehigherthanthoseoftheoriginalatomicorbitals. Threearebestdescribedasnonbondingmolecularorbitals,becausetheyhaveessentiallythesameenergyasthe3datomicorbitalsonthemetal. Ligand-fieldtheoryenablesthe3d,4s,and4porbitalsonthemetaltooverlapwithorbitalsontheligandtoformthe octahedralcovalentbondskeletonthatholdsthiscomplextogether.Atthe sametime,thismodelgeneratesasetoffiveorbitalsinthecenterof thediagramthataresplitintot2gandegsubshells,aspredictedbythecrystal-fieldtheory.Asaresult,wedon't havetoworryabout"inner-shell"versus"outer-shell"metalcomplexes. Ineffect,wecanusethe3dorbitalsintwodifferentways.Wecanusethemtoformthecovalentbond skeletonandthenusethemagaintoformtheorbitalsthatholdtheelectrons thatwereoriginallyinthe3dorbitalsofthetransitionmetal.