Electronic structure of diatomic molecules composed of a first-row transition metal and main-group element (h-f).
Understanding the nature of the transition-metal (TM)-main-group-element bond is important in many areas of science, such as organometallic chemistry,1,2 surface science,3 catalysis,4 high-temperature chemistry,5,6 and astrophysics.7,8 For example, the oxides are of interest to astrophysics as the constituents of cool stars and to surface science as zero-order models for the oxidation of a transition-metal surface.9 These systems are electronically complex and very difficult to treat theoretically. Indeed, while the use of quantum chemistry to obtain useful and reliable information about small organic molecules is now routine,11 a very different situation arises in the theoretical description of molecules containing a transition element. This is due to several factors, but the most important is that the extent of electron correlation required for even qualitatively correct results is significantly raised relative to molecules containing only main-group elements. The correlation between the electronic states of a molecule and the states of its constituent atoms has been an important concept in chemistry and physics for many years. For example, we know that if a molecule is composed of atoms that have large energy differences between their various electronic states, the molecule will be characterized by electronic states that are widely spaced, or granular. In the context of the preeminent orbital theory, the Hartree-Fock (HF) theory, this means that the molecular orbitals of molecules formed from these atoms will be widely spaced in energy, and the HF configuration will dominate the wave function around equilibrium. This is, of course, the reason the HF theory has achieved its unique role as both an interpretive and predictive tool in the chemistry of the firstand second-row, main-group elements. While the situation becomes somewhat flawed as one moves away from equilibrium structures, the conceptual simplicity and interpretability of the orbital picture can be preserved by constructing self-consistent wave functions in which the HF configuration is augmented by one or more configurations. This multiconfiguration, self-consistent field (MCSCF) approach is capable, in principle, of providing a uniform description of the evolution 679 Chem. Rev. 2000, 100, 679−716