Nonrelativistic and quasirelativistic ab‐initio pseudopotentials representing the Ne‐like X(Z−10)+ cores (X=Sc–Zn) of the first row transition metals and optimized (8s7p6d1f )/[6s5p3d1f ]‐GTO valence basis sets for use in molecular calculations have been generated. Excitation and ionization energies of the low lying states of Sc through Zn from numerical HF‐ as well as SCF‐ and CI(SD)‐pseudopotential calculations using the derived basis sets differ by less than 0.1 eV from corresponding all‐electron results.

Energy‐adjusted ab initio pseudopotentials for the first row transition elements

The chemistry of heterocyclic carbenes has experienced a rapid development over the last years. In addition to the imidazolin-2-ylidenes, a large number of cyclic diaminocarbenes with different ring sizes have been described. Aside from diaminocarbenes, P-heterocyclic carbenes, and derivatives with only one, or even no heteroatom within the carbene ring are known. New methods for the synthesis of complexes with N-heterocyclic carbene ligands such as the oxidative addition or the metal atom template controlled cyclization of β-functionalized isocyanides have been developed recently. This review summarizes the new developments regarding the synthesis of N-heterocyclic carbenes and their metal complexes.

Heterocyclic Carbenes: Synthesis and Coordination Chemistry

A set of f-polarization functions for pseudo-potential basis sets of the transition metals ScCu, YAg and LaAu

Some methods of describing electron correlation are compared from the point of view of requirements for theoretical chemical models. The perturbation approach originally introduced by Moller and Plesset, terminated at finite order, is found to satisfy most of these requirements. It is size consistent, that is, applicable to an ensemble of isolated systems in an additive manner. On the other hand, it does not provide an upper bound for the electronic energy. The independent electron-pair approximation is accurate to second order in a Moller-Plesset expansion, but inaccurate in third order. A series of variational methods is discussed which gives upper bounds for the energy, but which lacks size consistency. Finally, calculations on some small molecules using a moderately large Gaussian basis are presented to illustrate these points. Equilibrium geometries, dissociation energies, and energy separations between electronic states of different spin multiplicities are describe substantially better by Moller--Plesset theory to second or third order than by Hartree--Fock theory.

Theoretical models incorporating electron correlation

The nature of the bonding in transition-metal compounds.

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

Electronic structure of diatomic molecules composed of a first-row transition metal and main-group element (h-f).

The singlet-triplet gaps from STO-3G calculations for methylene and 29 substituted methylenes correlate with theoretical and empirical measures of ..pi.. donation by substituents. The ..pi.. donors stabilize the singlet more than the triplet, while ..pi.. acceptors have the opposite effect. Rough estimates of carbene singlet-triplet gaps can be made by using either calculated ..pi.. charges in the corresponding substituted benzene or empirical sigma/sub R//sup 0/ constants.

Carbene singlet-triplet gaps. Linear correlations with substituent. pi. donation

The major differential valence correlation effects of the lowest lying states arising from the s2dn, sdn+1, and dn+2 configurations of the first‐row transition metal atoms have been characterized using MCSCF and CI procedures. The important correlation effects are found to be, first, angular correlation of the 4s2 pair arising because of the near degeneracy of the 4s and 4p orbitals and, second, radial correlation of the 3d electron pairs. This large differential radial correlation of the 3d electrons can be interpreted as being due to nonequivalent d orbitals in the sdn+1 and dn+2 excited states. Both of these effects can be incorporated into a simple MCSCF wave function that reduces the error in the excited state atomic dissociation limits (∼0.2 eV in Sc–Cr and ∼0.5 eV in Mn–Cu for the sdn+1–s2dn excitation energy), yet still is of a form which lends itself easily to molecular calculations.

James F. Harrison

Papers

Electronic structure of diatomic molecules composed of a first-row transition metal and main-group element (h-f).

Carbene singlet-triplet gaps. Linear correlations with substituent. pi. donation

Valence correlation in the s2dn, sdn+1, and dn+2 states of the first‐row transition metal atoms

Electronic structure of methylene

Electronic structure of carbenes. I. Methylene, fluoromethylene, and difluoromethylene