Comproportionation of the palladium(0) compound Pd(PMe3)4 with the palladium(II) hexafluoroacetylacetonate compound Pd(hfac)2 or [Pd(hfac)(PMe3)2][hfac] gives the novel palladium (I) dimer [Pd2(PMe3)6][hfaC]2 (l). The analogous mixed-metal complex [PdPt(PMe3)6][hfac]2 (2) can be prepared similarly. The H-1 and P-31 NMR spectra are consistent with structures in which the two metal centers are connected by a metal-metal bond and the six terminal PMe3 ligands complete two mutually perpendicular square-planar coordination environments; the hfac groups are not attached to the metal centers and instead serve as counterions for the [M2(PMe3)6]2+ cations. This structure has been established by X-ray crystallography for the dipalladium(I) compound; the cation has 2-fold crystallographic symmetry, and the Pd-Pd distance is 2.598(1) angstrom. The average Pd-P distances to the equatorial PMe3 groups cis to the Pd-Pd bond are 2.342(l) angstrom while the Pd-P distances to the axial PMe3 groups trans to the Pd-Pd bond are slightly longer at 2.371(1) angstrom; the lengthening of the axial Pd-P bonds may be ascribable to the larger trans influence of the metal-metal bond. The axial phosphine ligands in 1 can be replaced by treatment with LiCl or NaI in tetrahydrofuran to afford the nonionic dinuclear complexes Pd2(PMe3)4Cl2 and Pd2(PMe3)4I2. Surprisingly, thermolysis of 1 at 100-degrees-C and 10(-3) Torr yields the palladium(II) methyl complex [PdMe(PMe3)3]-[hfac]; the methyl group attached to palladium in this product evidently arises via P-C bond cleavage of a PMe3 ligand. This result constitutes the first example of the isolation of a metal alkyl complex via P-C bond cleavage of a unidentate alkylphosphine. Analogously, thermolysis of 2 at 140-degrees-C and 10(-3) Torr yields a mixture of [PdMe-(PMe3)3][hfac] and the platinum(II) methyl complex [PtMe(PMe3)3][hfac]. These results are relevant to the incorporation of phosphorus and carbon impurities into metal films grown by metal-organic chemical vapor deposition from PMe3-containing precursors. Crystal data for 1 at -75-degrees-C : monoclinic, space group C2/c, a = 25.499(6) angstrom, b = 9.791(2) angstrom, c = 18.424(5) angstrom, beta = 106.85(2)-degrees, V = 4402(26) angstrom3, Z = 4,R(F) = 0.032, R(wF) = 0.033 for 241 variables and 3054 reflections with I > 2.58simga(I).