Studies of oxalamide and dithiooxalamide metal complexes. Part 5. Synthesis and crystal structure determinations of tetrabromo(N,N′-di-tert-butyloxalamide-OO′)tin(IV)-tetrahydrofuran ( 1/1 ) and tetrabromo(N,N′-dimethylsuccinamide-OO′)tin(IV)-tetrahydrofuran (1/1)