Several palladium polyselenides were prepared by the reactions of Pd2+ ions with Se-x(2-) (4 < x < 6) ligands in the presence of various cations. {(CH3)N(CH2CH2)(3)N}(2)[Pd(Se-6)(2)] (I) was prepared by the methanothermal reaction of K2PdCl4, K2Se4, and {(CH3)(3)N(CH2CH2)N(CH3)(3)}I-2 in a 1:3:2 molar ratio at 110 degrees C. Compound I crystallizes in the space group P2(1)/c with a 9.403(3) Angstrom, b = 15.050(5) Angstrom, c 10.807(4) Angstrom, beta = 91.73(3)degrees, and V = 1528.7(9) Angstrom(3). (H3NCH2CH2NH2)(2)[Pd(Se-5)(2)] (II) was prepared by the methanothermal reaction of PdCl2 Na2Se4, and (H3NCH2CH2NH3)Cl-2 in a 1:3:1 molar ratio at XO degrees C. Compound LI crystallizes in the space group P2(1)/n with a = 9.412(3) Angstrom, b = 8.671(3) Angstrom, c 12.666(3) Angstrom, beta 98.38(2)degrees, and V = 1023(1) Angstrom(3). The anions in [Pd(Se-5)(2)](2-) in II and [Pd(Se-6)(2)](2-) in I are polymeric and;possess layered structures. The organic cations are located between the layers. K-2(H3NCH2CH2NH3)(2)[Pd(Se-4)(2). 2Se(4)] (III) was prepared by the methanothermal reaction of PdCl2, K2Se4, and (H3NCH2CH2NH3)Cl-2 in a 1:3:1 molar ratio at 80 degrees C. Compound III crystallizes in the space group I2(1)2(1)2(1) with a = 12.587(9) Angstrom, b = 15.271(5) Angstrom, c = 15.404(9) Angstrom, and V = 2961(5) Angstrom(3). The [Pd(Se-4)(2). 2Se(4)](6-) anion in III is composed of a [Pd(Se-4)(2)](2-) three-dimensional adamantane-like framework with large cavities that are filled with the counterions and with uncoordinated Se-4(2-) ligands. K-6[Pd(Se-5)(4)] (IV) was prepared by the methanothermal reaction of K2PdCl4 and K2Se5 in a 1:6 molar ratio at 80 degrees C. The black polyhedral crystals of TV are slightly soluble in water. Compound IV crystallizes in the space group Pbcn with a 12.427(4) Angstrom, b = 13.777(9) Angstrom, c = 19.190(9) Angstrom, and V = 3285(5) Angstrom(3). The [Pd(Ses)(4)](6-) anion is molecular and one of the most Se-rich metal polyselenides known. It has a palladium center with four dangling Se-5(2-) ligands around it. Being a molecular fragment, it can be regarded as a building block for extended [Pd(Se-n)(2)](2-) frameworks. Optical spectroscopic and thermal gravimetric analysis data for the compounds are reported.