The action of Pd(AcO)2 on the imines 2,4,6-(CH3)3C6H2CH=N(CH 2)n-2′-RC6H4 (R = H, n = 0-2 (1a-c); R = CH3, n = 1 (1d)), in refluxing acetic acid, affords six-membered endo metallacycles possessing an aliphatic carbon-metal bond, in preference to four-, five-, or six-membered exo metallacycles with an aromatic carbon-metal bond. The five-membered exo metallacycles can be obtained by working under milder conditions and isomerize to the more stable six-membered endo metallacycles in refluxing acetic acid. The action of Pd(AcO)2 on the imines 2-CH3-3-R1-4-R2C6H 2CH=NC6H5 (R1 = H, R2 = CH3 (1f); R1 = CH3, R2 = CH3O (1g)) affords the five-membered endo metallacycles with an aromatic carbon-metal bond, but with the imine 2,5-(CH3)2C6H3CH=NC 6H5 (1e) the methyl group at carbon 5 prevents the metalation of the ortho carbon atom and the endo six-membered metallacycle with an aliphatic carbon-metal bond is formed. The reasons for the preference to form endo compounds and the high stability of six-membered derivatives containing Pd-C benzylic bonds are discussed.