Basis set and correlation effects on geometry of octahedral second-row transition-metal complexes

An extensive investigation of the basis-set effect on the predicted geometry of the redox pair [Ru(NH3)(6)](2+/3+) is presented. Basis sets where the core electrons have been replaced with a relativistic core potential as well as all-electron basis sets were tested. Best agreement with observations was obtained with the all-electron basis set MIDI augmented with a set of f-type polarization functions on the metal center. Other properties such as the vibration spectrum, the relative energy of the high-spin and low-spin states, and geometry changes upon oxidation/reduction of the central metal are discussed. The importance of electron correlation on the predicted geometry was estimated at the MP2, MP3, MP4(SDQ), CCSD, and CCSD(T) levels of theory. The MIDI(f) basis set is then used for other octahedral second-row transition-metal complexes and some other related complexes. The electronic spectrum of [Ru(NH3)(6)](2+) is also calculated using two different CI computational schemes. Surprisingly good agreement between the predicted electronic spectrum and the observed spectrum are obtained using one of the CI computational schemes. (C) 1996 John Wiley & Sons, Inc.