Selective oxidation of Mo(CO)(3) complexes of tritertiary 1,5,9-triphosphacyclododecane macrocycles (R(3)L) by halogens (X(2) = Cl-2, Br-2, I-2) to Mo(II) triphospha macrocycle complexes of the type (R(3)L)Mo(CO)(2)X(2) [R = (CH3)(2)CH (2), (CH3)(3)SiCH2 (3), C2H5 (4), (CH3)(2)CHCH2 (5)] allows the high yield and stereoselective liberation of the corresponding tritertiary macrocycles [syn,syn-R(3)L, R = (CH3)(2)CH (6), (CH3)(3)SiCH2 (7), C2H5 (8), (CH3)(2)CHCH2 (9)] in good yield (75-80%) by digestion in strong base. This method fails for the parent trisecondary macrocycle (H(3)L) and also for the intermediate Mo(II) salts, [(R(3)L)Mo(CO)(3)X]A(-) [X = halide, A(-) = halide, BPh(4) (1)]. Addition of halogen to (H(3)L)Cr(CO)(3) gives rise to the new blue-violet complexes (H(3)L)Cr(CO)(2)X(2) [X = Cl-2 (11), Br-2 (12)]. Paramagnetic susceptibilities indicate that 11 and 12 are low-spin d(4) six coordinate dicarbonyl halo-halide complexes of the type [(H(3)L)Cr(CO)(2)X]X. In this case, the trisecondary 1,5,9-triphosphacyclododecane (H(3)L, 13) may be liberated stereoselectively and in reasonable yield (60-70%) from 11 or 12. The macrocycles may alternatively be liberated from the Mo(II) dihalo complexes by action of CN-. The free trisecondary macrocycle can be alkylated nonstereoselectively to give the tritertiary macrocycles [syn,-anti-R(3)L; R = CH3 (24), C2H5 (8b), (CH3)(3)C (25)]. The inversion of phosphorus in the syn,syn isomer 8 to its syn,anti analogue, 8b, was shown to be slow at 156 degrees C. Exhaustive oxidation of the Mo(0) macrocycle complexes with H2O2 Or O-3 results in liberation of the corresponding macrocycle trioxides in good yield. All free macrocycles (and oxides) have been characterized by spectroscopic methods and as the hydrochlorides for selected ligands.