REACTIONS OF UNISOLABLE HOMOLEPTIC ARYLS OF NIOBIUM AND TANTALUM WITH PME2PH AND BUTNC - CRYSTAL-STRUCTURES OF M(2,5-ME2C6H3)2(ETA-2-2,5-ME2C6H2)(PME2PH)2, M = NB OR TA, AND TA(ETA-2-PHCNBUT)(ETA-2-PH2CNBUT)2(CNBUT)

The tetraaryls, MR4, M = Nb or Ta, R = 2-MeC6H4 or 2,5-Me2C6H3, made in situ by interaction of MCl4 and LiR or MgRBr react with PMe2Ph to give the eta-2-aryne compounds MR2(eta-2-aryne) (PMe2Ph)2. From the reaction of TaCl5 and LiPh in Et2O in the presence of PMe3 the similar tantalum(IV) benzyne, TaPh2(eta-2-C6H4) (PMe3)2, is obtained. Interaction of a green solution probably of TaPh5 with Bu(t)NC produces the tantalum(V) compound, Ta(PhCNBu(t)) (Ph2CNBu(t))2(CNBu(t)), via multiple insertion reactions of Bu(t)NC into the Ta-C bonds. The X-ray structures of M(2,5-Me2C6H3)2(eta-2-2,5-Me2C6H2) (PMe2Ph)2 (M = Nb or Ta) and Ta(eta-2-PhCNBu(t)) (eta-2-Ph2CNBu(t)) (CNBu(t)) have been determined. The isostructural arynes can be regarded as having a square-pyramidal structure with the C-C aryne bond occupying the axial site. The tantalum(V) compound contains one eta-2-iminoacyl and two eta-2-imine ligands.