The noncomplexed ligand Ph(2)PNHP(O)Ph(2) (HL) has been prepared, for the first time, from Ph(2)PNHPPh(2) (dppa) and H2O2 in moderate yield. On further treatment with H2O2 the phosphine oxide Ph(2)P(O)NHP(O)Ph(2) (1) can be formed. Reaction of [MCl(2)(COD)] (M = Pt or Pd; COD = cycloocta-1,5-diene) with 2 molar equiv of HL affords [MCl(2)(HL)(2)] (4, 5) in which both ligands are monodentate P-bound. The analogous dibromo and diiodo derivatives [MX(2)(HL)(2)] (6-9) were prepared from either [MX(2)(COD)] (M = Pt or Pd; X = Br or I) and HL reatment of [MCl(2)(HL)(2)] with an excess of NaI. Facile base deprotonation of the amine proton in 4 or 5 affords a new class of metallacycles [M(L)(2)] [L(-) = Ph(2)PNP(O)Ph(2)(-)] (10, 11) incorporating two five-membered M-P-N-P-O rings. Compound 10 was independently made from [Pt(CH3)(2)(COD)] and 2 equiv of HL in toluene at ambient temperature whereas the reaction of [Pt(CH3)Cl(COD)] with 2 equiv of HL affords solely trans-[Pt(CH3)Cl(HL)(2)] (13). Protonation of the neutral compound 10 with HCl in ethanol regenerates the ring-opened cis complex 4. In contrast, addition of HBF4 . OEt(2) to 10 (or 11) results in exclusive protonation at the nitrogen atom of the chelating ligands to give cis-[M{Ph(2)PNHP(O)Ph(2)-P,O}(2)][BF4](2) (15, 16); both chelate rings remain intact. The dicationic compounds 15, 16 can also be synthesized from 4, 5 in dichloromethane using Ag[BF4] as a chloride abstractor.All new compounds described have been characterized by a combination of multinuclear NMR spectroscopy, IR spectroscopy, and elemental analyses. The molecular structures of Ph(2)PNHP(O)Ph(2), cis-[PtBr2{Ph(2)PNHP(O)Ph(2)-P}(2)], cis-[Pt{Ph(2)PNP(O)Ph(2)-P, O}(2)], trans-[Pt(CH3)Cl{Ph(2)PNHP(O)Ph(2)-P}(2)] and cis-[Pt{Ph(2)PNHP(O)Ph(2)-P, O}(2)][BF4](2) have been determined by single-crystal X-ray diffraction. Crystals of HL are triclinic, space group P (1) over bar, with a = 10.410(2) Angstrom, b = 12.083(2) Angstrom, c = 9.036(4) Angstrom, alpha = 103.05(2)degrees, beta = 99.09(2)degrees, gamma = 72.98(1)degrees, V = 1054 Angstrom(3), and Z = 2. The final R and R(w) values were 0.061 and 0.047, respectively. Crystals of 6 are triclinic, space group P (1) over bar, with a = 13.113(4) Angstrom, b = 15.996(4) Angstrom, c = 13.011(4) Angstrom, alpha = 102.65(3)degrees, beta = 117.39(2)degrees, gamma = 101.03(2)degrees, V = 2225 Angstrom(3), and Z = 2. The final R and R(w) values were 0.039 and 0.031, respectively. Crystals of 10 are monoclinic, space group P2(1)/a, with a 17.600(13) Angstrom, b = 13.498(10) Angstrom, c = 18.641(9) Angstrom, beta = 105.13(1)degrees, V = 4274 Angstrom(3), and Z = 4. The final R and R(w) values were 0.041 and 0.036, respectively.Crystals of 13 are triclinic, space group P (1) over bar, with a = 14.686(2) Angstrom, b = 18.735(2) Angstrom, c = 9.018(3) Angstrom, alpha = 91.71(2)degrees, beta = 102.27(2)degrees, gamma = 68.54(1)degrees, V = 2253 Angstrom(3), and Z = 2. The final R and R(w) values were 0.041 and 0.044, respectively. Crystals of 15 are 002(1) Angstrom, c = 9.145(2) Angstrom, V = 2450 Angstrom(3), and Z = 2. The final R and R(w) values were 0.052 and 0.057, respectively. The P-bound monodentate complexes contain ligands with similar bond lengths to the neutral compound, while deprotonation/chelation results in lengthening of the P-O and shortening of the P-N bond length as a result of electronic delocalization.