Ab initio electronic structure calculations are used to study substituent effects in Vaska-type complexes, trans-IrL(2)(CO)X (1-X) (X = F, Cl, Br, I, CN, H, CH3, SiH3, OH, and SH; L = PH3). Both the electron affinity and the ionization potential of 1-X are computed to increase upon descending the halogen series of complexes, which indicates, surprisingly, that the complexes with more electronegative halogens are more difficult to reduce and easier to oxidize. The computed electron affinity trend is consistent with the half-wave reduction potential trend known for 1-X (L = PPh(3); X = F, Cl, Br, and I). Computed carbonyl stretch frequencies for 1-X are greater than experimental values (L = PPh(3)), but observed trends are well reproduced. The redox and spectroscopic trends are discussed in terms of the substituent effects on the electronic structure of 1-X, particularly as revealed in the molecular orbital energy level diagrams of these complexes. The reaction energy for H-2 addition to 1-X, leading to the cis,trans-(H)(2)IrL(2)(CO)X (2-X) product, has been computed. After electron correlation effects are included (MP4(SDTQ)), the reaction enthalpy computed for 1-Cl is -18.4 kcal/mol (L = PH3) as compared to a reported experimental value of -14 kcal/mol (L = PPh(3)). Compared with available experimental data, the electronic effects of L (L = PH3, NH3, or AsH3) and X on the thermodynamics of the H-2 addition reaction are accurately reproduced by the model calculations at all levels of theory (HF and MPn). Formation of the hypothetical products cis,trans- and trans,trans-(H)(2)IrL(2)(CO)X (2-X and 3-X) (X = BH2, NH2, and PH2) is used to demonstrate that pi-acceptor substituents promote the H-2 addition reaction to 1-X while pi-donor substituents disfavor addition.