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1.2.4.2YamamotocouplingreactionDiorganonickel(II)complexes,NiR2Lm,undergocouplingreactionstogiveR-RandhasbeenusedbyYamamotoet.al.forcouplingreactionsbetweenorganiccompounds[96,103].Theyhaveappliedthisreactionanduseditforthe21polycondensationofdihaloaromaticcompounds[104].Generalrepresentationsofthecouplingreactionsareshownbelow.whereLmisaneutralligand.Varioussolventscanbeusedforthecouplingreactionanditproceedsunderrelativelymildconditions.Itcanbeappliedtoawiderangeofaromaticcompounds.Itissimpleandpresentsaveryversatilemeansforthesynthesisofelectricallyconductingpolymersfromdihaloaromaticcompounds.ThehaloNi(II)complex,LmNiX(RX)isformedasthepolymerizationproceedsthroughtheoxidativeadditionoftheorganichalidetotheNi(0)complex.LmNi(RX)2isformedviathedisproportionationofthecomplexandthisyieldsX-Ar-Ar-Xonreductiveelimination.TheNi(0)catalystisformedfromthereductionofNi(II)inNiCl2byZn.Ageneralrepresentationofthecycleisillustratedinscheme1.5.Trans-Ni(II)intermediatecomplexesareusuallyformedwhentertiaryphosphineligandsuchasPPh3istheneutralligand[105].Cis-complexesareformedwhenbidentate2,2’-bipyridineistheneutralligand.Asreductiveeliminationismorefavouredbycistypecomplexes,acertainamountof2,2’-bipyridineisnormallyusedasapromoterofthepolymerizationprocess[106].Scheme1.5GeneralcatalyticcycleforYamamotocouplingYamamotocouplingreactionsmustbeconductedinadryandinertenvironmentasmoistureactsasachainterminatoranddeactivatesthecatalyst.Oxygenalsopoisonsthecatalystandsothereactionisnormallycarriedoutinnitrogen.Anexampleofitsuseinpolymersynthesisisshownbelowinscheme1.6.Scheme1.6(i)NiCl2,Zn,PPh3,bpy,DMF,reflux[95]