Active space, transition metal compounds

Transition metal compounds The CASSCF method has been used extensively to study compounds containing transition metals. The choice of the active space is almost never trivial for such systems and must be closely related to the chemical process under study. The CASSCF method is usually used together with the CASPT2 method (which will be described in detail below) to add dynamic correlation effects. That combination often makes it necessary to use a larger active spaces than one would need, for example, if one was combining CASSCF with MRCI calculations even if that is also non-trivial in many cases. 

Applications of the complete active space (CAS) SCF method and multiconfigurational second-order perturbation theory (CASPT2) in electronic spectroscopy are reviewed. The CASSCF/CASPT2 method was developed five to seven years ago and the first applications in spectroscopy were performed in 1991. Since then, about 100 molecular systems have been studied. Most of the applications have been to organic molecules and to transition metal compounds. The overall accuracy of the approach is better than 0.3 eV for excitation energies except in a few cases, where the CASSCF reference function does not characterize the electronic state with sufficient accuracy. 

Although the parent compound 1,2,3-triazole has not been studied, the crystal structure of benzotriazole (18) has been determined (74AX(B)1490). Benzotriazole crystallizes in the space group P2i. In association with its remarkable anticorrosion activity (Section 4.11.6.4.2) benzotriazole and its conjugate base form stable transition metal complexes whose crystal structures have, in part, been studied. Some examples are presented in Figures 5-7, with special regard to the molecular dimensions of the benzotriazole moiety. 

The preceding examples have indicated that the calculation of charge-transfer states in transition metal coordination compounds is certainly far from straightforward. However, a positive message is that, provided appropriate choices are made to keep the size of the CASSCF active space within limits, the subsequent CASPT2 are still accurate enough (with errors of at most 0.5 eV... 

Although the preferred catalyst is a composition of calcium, nickel, and potassium oxides in atomic ratios of 3.0 1.0 0.1, other catalysts of ternary compounds comprising alkaline earth, transition metal, and alkali oxides can be prepared in an active form. Oxidation of the catalyst is of primary importance. Methane space velocities of 4 mmol/g of catalyst/h are used at temperatures of 590 to 600 C. 

This review is intended to highlight any new reaction types that have been discovered, novel structural features that have been encountered, and chemical patterns and principles that have been observed for this class of compounds. Due to space limitations, only very selective examples of reactions are presented. Heterobimetallic species have been prepared and characterized for most combinations of iron and other transition metals. Their potential in the cooperative activation of small molecules, such as CO and unsaturated hydrocarbons, is an important research focus for this class of compounds. For mixed-metal clusters, enormous efforts have been made to develop rational cluster build-up methods and their potential applications in catalysis. Some important reviews that are pertinent to this type of species have appeared in the literature. 

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