The multistep synthesis of the novel diphosphine reference ligand L-2, 6, Ph2P(o-C6H4CH2C6H4-o)PPh2, has been streamlined and can be prepared on a ca. 20 g scale. It forms metallacycles with a variety of metal fragments. The resulting, and very rigid, boat-boat conformation forces a proton (H-endo) of the bridging methylene in close proximity to the metal, which in turn renders these protons (H-endo, H-exo) diastereotopic. The NMR spectra of [L2MCl2] [M = Pd, 9; M = Pt, 10] and of the organometallic derived compounds [L2Pd(PPh3)], 11, and [L2Pd(Cl)(eta(2)-CH2Ph)], 12, consist of a pair of doublets, with the H-endo coupled to the P of the metallacycle. The CH2-metal close proximity drives the electrophilic metalation of the bridging methylene by RhCl3 to form [L*Rh-2(Cl)(2)(MeCN)], 13, [L*(2) = Ph2P(o-C6H4CHC6H4-o)PPh2]. A significant example, which shows how the coordination number of the metal can affect the H-exo-H-endo resonance separation, is provided by the couple [L2Ni(C2H4)], 14, and [L2Ni(CO)(2)], 15. In order to show that the metallacycle size is crucial for the bridging methylene to function as a spectroscopic probe, we complexed the same [Fe(CO)(3)] fragment to L-2, 6, and L’(2), 17, [L’(2) = Ph2PO(o-C6H2(4,6Bu(2)(t))CH2(4,6B(2)(t))C6H2-o)OPPh2], to give [L2Fe(CO)(3)], 19, acid [L’Fe-2(CO)(3)], 18, respectively. In complex 17 the diastereotopic nature of these protons and hence the spectroscopic information was lost because of the presence of a 10-membered metallacycle. Crystal data : 9 is triclinic, space group P (1) over bar, a = 11.987(1) Angstrom, b = 15.990(2) Angstrom, c = 10.872(1) Angstrom, alpha = 91.42(1)degrees, beta = 111.01(2)degrees, gamma = 99.86(2)degrees, V = 1908.2(5) Angstrom(3), Z = 2, R = 0.053; 11 is monoclinic, space group C2/c, a = 39.071(5) Angstrom, b = 13.657(4) Angstrom, c = 19.848(5) Angstrom, beta = -92.45(2)degrees, V = 10581(5) Angstrom(3), Z = 8, and R = 0.047; 12 is triclinic, space group P (1) over bar, a = 11.289(1) Angstrom, b = 18.769(2) Angstrom, c = 11.077(1) Angstrom, alpha = 91.20(1)degrees, beta = 111.27(1)degrees, gamma = 105.26(1)degrees, V = 2107.7(4) Angstrom(3), Z = 2, and R = 0.039; 13 is orthorhombic, space group P2(1)2(1)2(1), a = 15.346(2) Angstrom, b = 18.188(3) Angstrom, c = 12.072(2) Angstrom, V = 3369.5(9) Angstrom(3), Z = 4, and R = 0.042.