The substitution of one of the ethylene ligands of the complexes Tp'Ir(C2H4)(2) (Tp' = Tp(Me2), 1*; Tp' = Tp, 1) by soft donors such as tertiary phosphines or carbon monoxide is a facile reaction that gives the corresponding Tp'Ir(C2H4)(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 PMe2Ph complex Tp(Me2)Ir(C2H4)(PMe2Ph) (3b*). Related hydride derivatives of Ir(III) can be obtained either by hydrogenation of the Ir(I) adducts (in general, this gives Tp'IrH2(L) compounds) or by thermal activation of one of the C-H bonds of the coordinated C2H4 ligand of the Tp(Me2)Ir(C2H4)(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](C2H4) to [Ir]H(CH=CH2) conversion does not require an overall change in the coordination mode of the Tp' ligand.