Valence-Bond-Theory-价键理论

ValenceBondTheoryAnothertopicthatyou’llneedtobefamiliarwithfortheSATIIChemistrytestisthatofvalencebondtheory.Bynow,youareawarethattwoatomswillformabondwhenthereisorbitaloverlapbetweenthem,andamaximumoftwoelectronscanbepresentintheoverlappingorbitals.Sincethepairofelectronsisattractedtobothatomicnuclei,abondisformed,andastheextentofoverlapincreases,thestrengthofthebondincreases.Theelectronicenergydropsastheatomsapproacheachother,butitbeginstoincreaseagainwhentheybecometooclose.Thismeansthereisanoptimumdistance,theobservedbonddistance,atwhichthetotalenergyisataminimum.Let’sdelvealittlemoredeeplyintosigmabondsnowanddescribetheminmoredetail.Asyouknow,sigma(s)bondsaresinglebonds.Theyresultfromtheoverlapoftwosorbitals,ansandaporbital,ortwohead-to-headporbitals.Theelectrondensityofasigmabondisgreatestalongtheaxisofthebond.Maximumoverlapformsthestrongest-possiblesigmabond,andthetwoatomswillarrangethemselvestogivethegreatest-possibleorbitaloverlap.Thisistrickywithporbitalssincetheyaredirectionalalongthex,y,andzaxes.Hybridorbitalsresultfromablendingofatomicorbitals(inotherwords,sandporbitals)tocreateorbitalsthathaveenergythat’sinbetweentheenergyoftheloneorbitals.Lookatthemethanemolecule,forexample:allfouroftheC—Hbondsare109.5oapart,whilenonbondedporbitalsareonly90oapart.Theorbitalsshownattheleftofthefigureareforanonbondedcarbonatom,butoncethecarbonatombeginstobondwithotheratoms(inthiscasehydrogen),theatomicorbitalshybridize,andthischangestheirshapeconsiderably.Noticehowthefirstsetoffiguresformthesp3atomicorbital,thehybrid,andthisleadstofurtherhybridization.Ammoniaalsohassp3hybridization,eventhoughithasalonepair.MultipleBondingNowlet’slookmorecloselyatpibonds.Aswementionedearlierinthischapter,pi(p)bondsresultfromthesidewaysoverlapofporbitals,andpiorbitalsaredefinedbytheregionaboveandbelowanimaginarylineconnectingthenucleiofthetwoatoms.Keepinmindthatpibondsneveroccurunlessasigmabondhasformedfirst,andtheymayformonlyifunhybridizedporbitalsremainonthebondedatoms.Also,theyoccurwhensporsp2hybridizationispresentoncentralatombutnotsp3hybridization.Below,weshowtheformationofasetofsp2orbitals.Thismoleculewouldcontainadoublebond,likeethene.Noticeagainhowthefirstsetoffiguresformthesp2atomicorbital,thehybrid,andthelastfigureshowsfullhybridization:Thesetofporbitalsthatareunhybridizedarenotshowninthisdepiction:Adifferentview,whichdoesn’tshowthehydrogensandcentersontheCatoms,showstheunhybridizedporbitalsthatcreatethesidewaysoverlapthat’snecessarytocreatethedoublepibond:Here’showitlookswithallthepiecesputtogether:Hereisatablesummarizinghybridizationandstructure:effectivepairshybridizationgeometry2splinear3sp2trigonalplanar4sp3tetrahedral5dsp3trigonalbipyramidal6d2sp3octahedral