Novel half-sandwich 18ē and 16ē arene ruthenium iminophosphonamide complexes [(η6-C6Me6)RuCl{(R'N)2PR2}] (3a-c) and [(η6-C6Me6)Ru{(R'N)2PR2}]+(X-) (4a-c) (a, R = Ph, R' = p-Tol; b, R = Et, R' = p-Tol; c, R = Ph, R' = Me. X = BF4, PF6 or BArF4) were synthesized. The elongated Ru-Cl bond in the 18ē complexes is shown to dissociate even in apolar solvents to form the corresponding 16ē cations, which can be readily isolated as salts with non-coordinating anions. The coordinatively unsaturated 16ē complexes are stable species due to efficient π-electron donation from the nitrogen atoms of the zwitterionic NPN-ligand. The ruthenium iminophosphonamides are moderately active in the ROMP polymerization of norbornene; the 16ē complexes 4a,b yield high molecular weight polymers (Mn∼ 300 × 103) with a narrow distribution Mw/Mn∼ 1.6, while the 18ē complexes 3a,b give polymers of lower molecular weight (Mn < 50 × 103) with a wider polydispersity index Mw/Mn∼ 2.5.