Selected secondary phosphanes (H-PR2; R = Ph, Cy, (i)Pr) smoothly react with a parent amido-bridged diiridium cyclooctadiene complex affording mixed amido/(bis)phosphido dinuclear species. A careful investigation of the reaction profile, carried out by experimental and theoretical tools, revealed that, after an initial amido/phosphido exchange, at low temperatures a second molecule of secondary phosphane adds to the dinuclear system through an oxidative addition process leading to a hydrido amido/bis(phosphido) mixed-valence complex [Ir(III)/Ir(I)]. These species rearrange above -10 °C into the most stable isomer that arises from a migration of the hydrido moiety to one of the =CH fragments of a coordinated cod molecule, a transformation facilitated by the formation of an intermetallic bond. Further heating of these species reductively eliminates ammonia affording bis(phosphido)-metal-metal bonded complexes.