Synthesis and study of the benzyl- and naphthylpalladium(IV) complexes PdBrMe2(CH2Ar)(L2) (L2 = bpy, phen) and .mu.-hydrocarbyl palladium(IV)-palladium(IV) and palladium(IV)-platinum(IV) complexes and the structure of fac-PdBrMe2(CH2-p-C6H4Br)(phen)