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 the metal centre in trans-[(Ph3P)2Pd(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 structures split in two isostructural and isomorphous groups, namely orthorhombic 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 trans influence of X within these pairs follows the trend Cl > F and I > Br. However, the trans influence of Cl is slightly stronger than that of Br. Both structural and C-13 NMR studies revealed that electron-donating effects of (Ph3P)(2)PdX increase along the series X = I < Br < Cl < F, in accord with the previously reported 'halogen anomaly' phenomenon. 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 possible 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 the solvent smoothly to give a mixture of CH2ClF and CH2F2 in high yield. When chloroform was used instead of CH2Cl2, dichlorocarbene was generated slowly, forming the corresponding cyclopropane in the presence of styrene. All observations were rationalized successfully in terms of the filled/filled effect and push/pull interactions. (C) 1998 Elsevier Science S.A. All rights reserved.