The thermolyses of dihalobis(1-phenyl-3,4-dimethylphosphole)palladium(II) complexes [(DMPP)(2)PdX(2), X = Cl, Br, I] were investigated in 1,1,2,2-tetrachloroethane solutions at 145 degrees C and in the crystalline state at 140 OC. For cis-(DMPP)(2) PdCl2 and cis- or trans-(DMPP)(2) PdBr2 four types of products were formed : (1) [4 + 2] cycloaddition products, (2) [2 + 2] cycloaddition products, (3) compounds that result from 1,5-hydrogen migration from a methyl group on one phosphole to the beta-carbon of an adjacent phosphole (exo-methylene), and (4) products that result from an intermolecular [4 + 2] coupling of two phospholes followed sequentially by phosphinidene elimination and intramolecular [4 + 2] cycloaddition to another phosphole to give diphosphatetracyclotetradecatrienes (DPTCT). trans-(DMPP)(2)PdBr2 undergoes thermal isomerization to cis-(DMPP)(2)PdBr2 in the solid state, and cis- and trans-(DMPP)2PdBr2 give the same products in both their solid- and solution-slate thermolyses. In contrast, trans-(DMPP)(2) PdI2 neither isomerizes to the cis-isomer nor undergoes any of the phosphole coupling reactions in either the solution or solid state.The crystal structures of trans-(DMPP)(2)PdX(2) (X = Br, I), {(DMPP)(2)[2 + 2]}PdBr2, {(DMPP)(2)(exo-methylene)}PdBr2, and (DPTCT)PdCL(2) were determined. They crystallize in the monoclinic P2(1)/c, triclinic P (1) over bar, monoclinic P2(1)/c, monoclinic P2(1)/n, and orthorhombic P2(1)2(1)2(1) space groups in units cells of the following dimensions: a 10.158 (3) Angstrom, b = 14.876 (4) Angstrom, 16.829 (5) Angstrom, beta = 104.25 (2)degrees, rho(calc) 1.732 g/cm(3), Z = 4; a = 9.025(1) Angstrom, b = 11.023 (1) Angstrom, c 13.833 (1) Angstrom, alpha = 101.15 (1)degrees, beta = 98.82 (1)degrees, gamma = 105.30 (1)degrees, rho(calc) = 1,886 g/cm(3), Z = 2; a = 13.090 (2) Angstrom, b = 17.637 (2) Angstrom, c = 21.834 (2) Angstrom, beta = 100.51 (1)degrees, rho(calc) = 1.738 g/cm(3), Z = 4, a = 10.721 (1) Angstrom, b = 16.929 (1) Angstrom, c = 14.675(1) Angstrom, beta = 97.86 (1)degrees, rho(calc) = 1.663 g/cm(3), Z = 4; and a = 15.532 (3) Angstrom, b = 19.401 (4) Angstrom, c = 9.910 (2) Angstrom, rho(calc) = 1.490 g/cm(3), Z = 2, respectively. Least-squares refinements converged at final values of R(F) of 0.041, 0.0354, 0.0624, 0.0533, and 0.035 for 2770, 2672, 2729, 2159, and 2525 independent observed reflections, respectively. Kinetic studies suggest that the reaction mechanisms are the same in both the solid and solution states and that the reaction mechanisms are substantially different from those previously reported for the thermolyses of the analogous cis-(DMPP)(2)PtX(2) complexes.