Reaction of the lithium-bis(iminophosphoranyl)methanide compounds Li[CH(PPh2=N-C6H4-R′-4)2] (2a, R′ = CH3; 2b, R′ = CH3 and NO2; 2c, R′ = NO2) with [ML2Cl]2 (M = Rh, L2 = COD, L = CO; M = Ir, L2 = COD) yields the novel bis (iminophosphoranyl) methanide complexes [M{CH-(PPh2=N-C6H4-R′-4) 2}L2] (3a-d, 4a-c), in which the ligand is coordinated as a σ-N,σ-C chelate. Complex 3a (M = Rh, R′ = CH3, L2 = COD) has also been synthesized in the reaction of CH2-(PPh2=N-C6H4-CH 3-4)2 (1a) with [Rh(COD)(OMe)]2. The COD ligand in the Ir-COD compounds 4a-c can be substituted by reaction with CO to give [Ir{CH(PPh2=N-C6H4-R′) 2}(CO)2] (4d-f). The solid-state structure of 4a (M = Ir, R′ = CH3, L2 = COD) has been determined by an X-ray diffraction study. Compound 4a crystallizes in the triclinic space group P1 with a = 10.633 (2) A?, b = 11.766 (2) A?, c = 17.366 (2) A?, α = 99.73 (1)°, β = 90.59 (3)°, γ = 95.42 (2)°, and Z = 2. The structure was refined to R = 0.055 (Rw = 0.060) using 4065 independent observed reflections. The bis-(iminophosphoranyl)methanide ligand is coordinated in a σ-N,σ-C chelate mode toward the square planar Ir, giving rise to a four-membered M-N-P-C metallacycle and one pendant iminophosphorane moiety. Important bond distances and angles are P1-N1 = 1.55 (1) A?, P2-N2 = 1.62 (1) A?, P1-C1 = 1.82 (1) A?, P2-C1 = 1.77 (1) A?, Ir-C1 = 2.16 (1) A?, and Ir-N2 = 2.07 (1) A?, and N2-Ir-C1 = 73.4 (4)°, C1-P1-N1 = 119.3 (6)°, and C1-P2-N2 = 96.4 (5)°. In solution the bis(iminophosphoranyl)-methanide complexes undergo a dynamic process, in which both halves of the ligand and both coligands become equivalent. The exchange processes for 3a, 3b, 4a, and 4d have been studied by using variable-temperature 1H and 31P NMR; they occur via a Berry pseudorotation, involving an intermediate five-coordinate complex that is formed via intramolecular attack of the pendant iminophosphorane entity. Additional structural information has been acquired from IR and 13C, 103Rh, and solid state 31P{1H} CP/MAS NMR spectroscopy. Complexes 3a, 3b, 4a, and 4d react with CO2 in an aza-Wittig reaction to give the (iminophosphoranyl) (oxophosphoranyl) methyl complexes [M{CH(PPh2=N-C6H4-CH3-4)(PPh 2=O)}L2](5a-d), p-tolyl isocyanate, and di-p-tolylcarbodiimide. The inertness of the four membered M-N-P-C ring toward CO2 stresses the nonreactivity of this polarized metallacycle. Treatment with CF3COOH results in direct protonation of the nitrogen atom of the free iminophosphorane moiety, affording the complexes [M{CH-(PPh2=N-C6H4-CH3-4)(PPh 2-NH-C6H4-CH3-4)}L2] +X- (II). Reaction with HCl leads to a net protonation of either the methanide or the nitrogen atoms with formation of the known coordination complexes of bis(iminophosphoranyl)methane, in which the ligand is either coordinated as a σ-C chelate (in II) or as a σ-N,σ-N′ chelate in [M{(4-CH3-C6H4-N=PPh2) 2CH2}-L2}+X- (III), respectively. This reaction proceeds via initial oxidative addition of HCl to Rh or Ir and a subsequent hydrogen-migration reaction from the metal to either the methanide C atom or the N atom of the coordinated iminophosphorane entity. This has been evidenced by the observation of Ir-hydride complexes (1H NMR) in the reaction of 4a with 1 equiv of HCl at 193 K. These complexes react further to give II and III upon raising the temperature to 233 K. Addition of excess HX (X = CF3COO, Cl) to the bis(iminophosphoranyl)methanide complexes 3a, 3b, 4a, and 4d gives [ML2X]2 and the amino[(aminophosphonio)methyl]phosphonium compounds [CH2-(PPh2-NH-C6H4-CH 3-4)2]2+·2X- (IV).