The two chelating ligands 2,2-dimethyl-1,3-bis(diphenylphosphino)propane (2,2-dm-1,3-dppp) and 2,2-dimethyl-1,3-bis(diphenylarsino)propane (2,2-dm-1,3-dpap) have been synthesized and used to generate the new complexes Mo(CO)4(L-L) W(CO)4(L-L), and XMn(CO)3(L-L) where X = Cl, Br, I, CH3, and CH3CO. A rapid chair-chair interconversion of the six-membered metal chelate ring system in the molybdenum and tungsten complexes results in the apparent equivalence of the phenyl groups (13C NMR), methyl groups (1H and 13C NMR), and methylene protons (1H NMR). The assignment of a facial configuration to each of the manganese complexes is based on infrared spectra in the 2000-cm−1 region, a single 31P resonance for complexes of 2,2-dm-1,3-dppp, and the inequivalence of the 2-methyl groups (1H and 13C NMR), the phenyl groups (13C NMR), and the geminal methylene hydrogens (1H NMR). The 1H and 13C NMR spectra of the manganese complexes are interpreted on the basis of a single preferred chair conformation. Specific assignments for the 1H and 13C NMR resonance signals of the dissimilar 2-methyl groups are based upon the observation of four-bond planar W-coupling of the equatorial 2-methyl protons to phosphorus (4JHP ≃ 3 Hz) and Karplus-type three-bond coupling between phosphorus and the equatorial 2-methyl carbon (3JCP ≃ 10.5 Hz). Phosphorus coupling to the axial 2-methyl carbon (4JCP = 2.5 Hz) and protons is considerably weaker. © 1982, American Chemical Society. All rights reserved.