THE TRANS INFLUENCE OF F, CL, BR AND I LIGANDS IN A SERIES OF SQUARE-PLANAR PD(II) COMPLEXES - RELATIVE AFFINITIES OF HALIDE ANIONS FOR THEMETAL CENTER IN TRANS-[(PH3P)(2)PD(PH)X]

Single crystal X-ray diffraction studies of trans-[(Ph3P)(2)Pd(Ph)X] ( X = F (1), Cl (2), Br (3), and I (4)) were carried out. The four struc tures split in two isostructural and isomorphous groups, namely orthor hombic for 1 and 2 (space group Pbca, Z = 8) and triclinic for 3 and 4 (space group P-1, Z = 2). According to the Pd-C bond length, the tran s influence of X within these pairs follows the trend Cl > F and I > B r. However, the trans influence of Cl is slightly stronger than that o f Br. Both structural and C-13 NMR studies revealed that electron-dona ting effects of (Ph3P)(2)PdX increase along the series X = I < Br < Cl < F, in accord with the previously reported 'halogen anomaly' phenome non. Relative affinities of X- for the Pd centre in [(Ph3P)(2)Pd(Ph)]( +) were studied by P-31 NMR in rigorously anhydrous CH2Cl2 solutions, and equilibrium constants and Delta G values were obtained for all pos sible combinations. The sequence F- > Cl- > Br- > I- is characteristic of halide preference for the Pd complexes. Dissolving 1 and PPN Cl in dry CH2Cl2 resulted in the release of 'naked' F- which fluorinated th e solvent smoothly to give a mixture of CH2ClF and CH2F2 in high yield . When chloroform was used instead of CH2Cl2, dichlorocarbene was gene rated slowly, forming the corresponding cyclopropane in the presence o f styrene. All observations were rationalized successfully in terms of the filled/filled effect and push/pull interactions. (C) 1998 Elsevie r Science S.A. All rights reserved.