The structure of d6 IrIIIL2ZZ'X complexes (L = PH3; Z, Z' = H-, Me-, Cl-, X = H-, Cl-, NH2-, OR- (R = H, CH3, CH2F, CF3)) has been studied by ab initio pseudopotential calculations. In agreement with a previously published EHT study (Rachidi, I. E.-I.; Eisenstein, O.; Jean, Y. New J. Chem. 1990, 14, 671), it is shown that two structures are possible for a closed-shell state: a square-based pyramid (SP also called T) and a distorted trigonal bipyramid (dist-TBP also called Y) in which two ligands subtend an acute angle of about 70-80-degrees. It is shown that the Y structure is favored when the ligand in the site trans to the acute angle is both a weak sigma-donor and a good pi-donor (X = Cl-, NH2-, OR-). These results are in full agreement with known experimental data: Ir(PCy3)2ClH(C5H6) (2) and Ir[N(SiMe2CH2P(Ph2)2)]RR' (3). The calculations also indicate that the structure of the recently synthesized Ir(PCy3)2(OCH2CF3)H2 complex, 4b, in which the H centers could not be located in the X-ray data should be of the same Y type. We show that a partial metal-X pi-bond appears in the Y structure involving the lone pair of X and an empty metal orbital. This shortens the M-X bond significantly. When X is a single-faced donor (X = NH2-, OR-), a preferred orientation of X is expected. Rotational barriers, calculated with X = NH2- and OR-, have been shown to follow the electron-releasing ability of X. We show that the requirement of the metal for this electron donation from X is sufficient to stabilize the most sterically hindered isomer such as 4b. The fluxional behavior of the different complexes is discussed. The structure of the lowest triplet state, which is much higher in energy than the ground singlet state, is also determined in the case of the simplest complexes.