Oxidation of PdMe2(L-2) [L-2 = = 2,2'-bipyridine (bpy), 1,10-phenanthroline (phen)] by diphenyl diselenide provides I the first examples of stable dimethylpalladium(IV) complexes PdMe2(SePh)(2)(L-2), and pallada (IV)cyclic Pd(CH2 CH2CH2CH2)(SePh)(2)(bpy) may be similarly isolated. X-ray structural studies of the octahedral dimethylpalladium(IV) complexes and their isomorphous platinum(IV) analogues have been completed [L-2 = bpy, orthorhombic Pnma; L-2 = phen, triclinic P (1) over bar; an additional phase for PtMe2(SePh)(2)(phen), tetragonal, I4(1)/a]. The complexes PdMe2(SePh)(2)(L-2) decompose at moderate temperatures in CDCl3 following first-order behavior [L-2 = bpy, E-a similar to 46 kJ mol(-1), Delta S double dagger(20 degrees C) similar to -170 J K-1 mol(-1); L-2 phen, E-a similar to 36 kJ mol(-1), Delta S double dagger(20 degrees C) similar to -204 J K-1 mol(-1)] to give ethane and Se(Ph)Me, together with small quantities of SePh2. Similar C ... C, C ... O, C ... S, and C ... Se bond formation processes occur on decomposition of palladium(IV) species that are too unstable to be isolated on the oxidation of PdMe2(bpy) or Pd(CH2CH2CH2CH2)(bpy) by (O2CPh)(2) or (SPh)(2).