The substitution of one of the ethylene ligands of the complexes Tp'Ir(C(2)H(4))(2) (Tp' = Tp(Me)()2, 1; Tp' = Tp, 1) by soft donors such as tertiary phosphines or carbon monoxide is a facile reaction that gives the corresponding Tp'Ir(C(2)H(4))(L) adducts. Spectroscopic studies support their formulation as five-coordinate, 18-electron species that possess a distorted trigonal bipyramidal geometry. This proposal has been confirmed by a single-crystal X-ray study carried out with the PMe(2)Ph complex Tp(Me)()2Ir(C(2)H(4))(PMe(2)Ph) (3b). Related hydride derivatives of Ir(III) can be obtained either by hydrogenation of the Ir(I) adducts (in general, this gives Tp'IrH(2)(L) compounds) or by thermal activation of one of the C-H bonds of the coordinated C(2)H(4) ligand of the Tp(Me)()2Ir(C(2)H(4))(L) compounds. All these reactions can be understood by invoking the participation of transient, 16-electron (eta(2)-Tp')Ir intermediates, but the thermodynamics of the [Ir](C(2)H(4)) to [Ir]H(CH=CH(2)) conversion does not require an overall change in the coordination mode of the Tp' ligand.