TRANS INFLUENCE OF PHOSPHINES ON DIMER-MONOMER INTERCONVERSION OF 2-PYRIDINETHIOLATE COMPLEXES - STRUCTURES OF [PD(MU-ETA(2)-PYS-N,S)CL(L)](2) (L=PME(2)PH, PMEPH(2)) AND PD(ETA(2)-PYS)CL(PPH(3))

The molecular structures of [Pd(mu-eta(2)-pyS-N,S)Cl(PMe(2)Ph)](2) (4) and [Pd(mu-eta(2)-pyS-N,S)Cl(PMePh(2))](2) (5) have been determined b y X-ray diffraction. Crystallographic data: for 4, monoclinic P2(1)/n, Z = 4, a = 8.952(6) Angstrom, b = 17.665(13) Angstrom, c = 19.288(12) Angstrom, beta = 95.87(5)degrees, V = 3034(4) Angstrom(3); for 5, mon oclinic C2/c, Z = 4, a 20.886(10) Angstrom, b = 12.422(6) Angstrom, c = 15.394(14) Angstrom, beta = 111.62(5)degrees, V = 3713(4) Angstrom(3 ). The structural results indicate that there is little variation in t he bonding between the pyS ligand and the palladium centers in the dim eric complexes. The molecular structure of Pd(eta(2)-pyS)Cl(PPh(3)) (3 ) has been redetermined in a centrosymmetric space group. Crystallogra phic data: for 3, triclinic P ($) over bar 1, Z = 2, a = 9.492(4) Angs trom, b = 10.251(6) Angstrom, c = 12.964(8) Angstrom, alpha = 74.29(5) degrees, beta = 72.66(4)degrees, gamma 65.48(3)degrees, V = 1079.7(10) Angstrom(3). In solution, complexes 4 and 5 establish equilibria with the corresponding monomer complexes, Pd(eta(2)-pyS)Cl(PMe(2)Ph) (6) a nd Pd(eta(2)-pyS)Cl(PMePh(2)) (7), respectively. While the monomeric c omplexes are entropically favored, there is an enthalpic preference fo r the dimeric complexes since coordination of pyS in the bridging mode does not require the severe distortion necessary for chelation of the ligand to a single metal center, The interconversion of 4 and 6 has b een studied by variable-temperature P-31{H-1} NMR spectroscopy. From t he changing ratios of the integrated intensities of the signals for 4 and 6 over the 0-70 degrees C temperature range, values of Delta H = 4 6 +/- 2 kJ mol(-1) and Delta S = 109 +/- 10 J K-1 mol(-1) were calcula ted for the dimer-monomer interconversion, The large positive value ob served for Delta S strongly supports that 4 undergoes reversible disso ciation into two monomers. Dilution experiments provide additional ver ification that the observed dynamic processes involve dimer-monomer in terconversions, A shift in equilibrium position toward the monomeric c omplexes with increasing aryl substitution can be seen through compari son of P-31{H-1} NMR spectra of equimolar amounts of [Pd(mu-eta(2)-pyS -N,S)Cl(PMe(3))](2) (1), 4, and 5 dissolved in toluene-d(8) at 20 degr ees C. Comparison of the energetics determined for the 1-Pd(eta-pyS)Cl (PMe(3)) (2) and 4-6 equlibria indicates that this shift is due to the decreasing values of Delta H upon aryl substitution of the phosphines , It is concluded that the differing values for the Delta H for the di mer-monomer equilibria arise from strengthened Pd-N interactions resul ting from the decreased donor ability of the trans aryl-substituted ph osphines in the monomeric complexes.