AB-INITIO MOLECULAR-ORBITAL STUDY OF SUBSTITUENT EFFECTS IN VASKA TYPE COMPLEXES (TRANS-IRL(2)(CO)X) - ELECTRON-AFFINITIES, IONIZATION-POTENTIALS, CARBONYL STRETCH FREQUENCIES, AND THE THERMODYNAMICS OF H-2 DISSOCIATIVE ADDITION

Ab initio electronic structure calculations are used to study substitu ent effects in Vaska-type complexes, trans-IrL(2)(CO)X (1-X) (X = F, C l, Br, I, CN, H, CH3, SiH3, OH, and SH; L = PH3). Both the electron af finity and the ionization potential of 1-X are computed to increase up on descending the halogen series of complexes, which indicates, surpri singly, that the complexes with more electronegative halogens are more difficult to reduce and easier to oxidize. The computed electron affi nity 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 st retch frequencies for 1-X are greater than experimental values (L = PP h(3)), but observed trends are well reproduced. The redox and spectros copic 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 ar e 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 therm odynamics of the H-2 addition reaction are accurately reproduced by th e model calculations at all levels of theory (HF and MPn). Formation o f 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 p i-acceptor substituents promote the H-2 addition reaction to 1-X while pi-donor substituents disfavor addition.