Novel 18e̅ and 16e̅ pentamethylcyclopentadienyl rhodium­(III) complexes [(η5-C5Me5)­RhX­(NPN)] (1a,b, X = Cl; 2a–c, X = PF6, BAr4 F) with chelating zwitterionic iminophosphonamide (NPN) ligands (Ph2P­(NR)­(NR′); a, R = R′ = p-Tol; b, R = p-Tol, R′ = Me; c, R = R′ = Me) were synthesized and characterized by single-crystal X-ray diffraction. In the 16e̅ complexes 2, the rhodium (Rh) atom is efficiently stabilized by π-donation of unshared N electrons, thus hampering coordination of the external ligands and rendering the 18e̅ complexes labile. Due to low coordination enthalpy, the cationic 18e̅ monocarbonyl and pyridine adducts 2a·L are stable only at low temperatures. At room temperature, 2·CO adducts readily give stable carbonyl–carbamoyl complexes [(η5-C5Me5)­Rh­(CO)­{(CO­(NR′)­Ph2P­(NR)}]+ (4) formed as a result of CO insertion into the Rh–N bond, thus showing high nucleophilicity of the N atoms in 18e̅ complexes. High basicity of the Na+NPN– precursors caused side deprotonation of the η5-C5Me5 ligand during the synthesis of 1 that yields unstable fulvene Rh­(I) complexes [(η4-C5Me4CH2)­Rh­{Ph2P­(NR)­(NR′)2}] (3a,b). Complex 3a undergoes a facile reaction with isoprene to yield an unusual [(η5:η1-C5Me4(CH2)­C­(Me)CHCH2)­Rh­(NPN)] complexthe first example of intermolecular 1,4-metallacycloaddition of diene to the Rh–fulvene complex.