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 by 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, monoclinic 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 the bonding between the pyS ligand and the palladium centers in the dimeric complexes. The molecular structure of Pd(eta(2)-pyS)Cl(PPh(3)) (3) has been redetermined in a centrosymmetric space group. Crystallographic data : for 3, triclinic P ($) over bar 1, Z = 2, a = 9.492(4) Angstrom, 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) and Pd(eta(2)-pyS)Cl(PMePh(2)) (7), respectively. While the monomeric complexes are entropically favored, there is an enthalpic preference for 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 been studied by variable-temperature P-31{H-1} NMR spectroscopy. From the 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 = 46 +/- 2 kJ mol(-1) and Delta S = 109 +/- 10 J K-1 mol(-1) were calculated for the dimer-monomer interconversion, The large positive value observed for Delta S strongly supports that 4 undergoes reversible dissociation into two monomers. Dilution experiments provide additional verification that the observed dynamic processes involve dimer-monomer interconversions, A shift in equilibrium position toward the monomeric complexes with increasing aryl substitution can be seen through comparison 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 degrees 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 dimer-monomer equilibria arise from strengthened Pd-N interactions resulting from the decreased donor ability of the trans aryl-substituted phosphines in the monomeric complexes.