Mechanistic studies of olefin polymerizations catalyzed by aryl-substituted R-diimine-Pd(II) complexes are presented. Syntheses of several cationic catalyst precursors, [(N ∧ N)Pd(CH 3 )(OEt 2 )]BAr′ 4 (N ∧ N ) aryl-substituted R-diimine, Ar′ ) 3,5-(CF 3 ) 2 C 6 H 3 ), are described. X-ray structural analyses of [ArNd C(H)C(H)dNAr]Pd(CH 3 )(Cl) and [ArNdC(Me)C(Me)dNAr]Pd(CH 3 ) 2 (Ar ) 2,6-(iPr) 2 C 6 H 3 ) illustrate that o-aryl substituents crowd axial sites in these square planar complexes. Low-temperature NMR studies show that the alkyl olefin complexes, (N ∧ N)Pd(R)(olefin) + , are the catalyst resting states and that the barriers to migratory insertions lie in the range 17-19 kcal/mol. Following migratory insertion, the cationic palladium alkyl complexes (N ∧ N)Pd(alkyl) + formed are β-agostic species which exhibit facile metal migration along the chain ("chain walking") via β-hydride elimination/readdition reactions. Model studies using palladiumn-propyl and -isopropyl systems provide mechanistic details of this process, which is responsible for introducing branching in the polyethylenes made by these systems. Decomposition of the cationic methyl complexes (ArN ∧ NAr)Pd(CH 3 )(OEt 2 ) + (Ar ) 2,6-(iPr) 2 C 6 H 3 , 2-tBuC 6 H 4 ) occurs by C-H activation of β-C-H bonds of the ortho isopropyl and tert-butyl substituents and loss of methane. The rate of associative exchange of free ethylene with bound ethylene in (N ∧ N)Pd(CH 3 )(C 2 H 4 ) + is retarded by bulky substituents. The relationship of these exchange experiments to chain transfer is discussed.

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The complexes [(TMS)2pda](NPh)WR2 ((TMS)2pda = o-(Me3SiN)2C6H4}2-; R = CH3 (1), CH2CMe3 (4)) react with tBuNC to form the octahedral monoadducts [(TMS)2pda](NPh)WR2(CNtBu) (1a and 4a). 1a is observed only at low temperature, while 4a can be isolated. Both 1a and 4a react with tBuNC to form the corresponding trigonal bipyramidal bis(η2-imino-acyl) species [(TMS)2pda](NPh)W[η2-(tBu)NCR]2 (R = CH3 (2), CH2CMe3 (5)), which have been isolated and characterized. Compound 2 isomerizes upon heating to the square pyramidal metallacycle [(TMS)2pda](NPh)W[(tBu)NC(CH3)C(CH3)N(tBu)] (3), which has been isolated as a red solid. X-ray crystal structures have been obtained for compounds 2 and 3. Crystals of 2 are orthorhombic, space group P212121, with a = 9.1175(3) Å, b = 17.9587(5) Å, c = 20.9435(6) Å, and Z = 4. Compound 2 has trigonal bipyramidal geometry with the imido N and one amido N in axial positions. Crystals of 3 are monoclinic, space group C2/c, with a = 35.1302(9) Å, b = 12.6459(2) Å, c = 16.2386(6) Å, β = 112.021(3)°, and Z = 8. 3 has distorted square pyramidal geometry with the imido moiety in the apical position.

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