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The cationic (C2H4)M+ complexes (M = Cu, Ag, and Au) have been examined by different ab initio molecular orbital, density functional (DFT), and density functional/Hartree−Fock (DFT/HF) hybrid methods using relativistic effective core potentials and a quasi-relativistic approach to account for relativistic effects. For (C2H4)Au+ a substantial relativistic stabilization is observed, such that the computed binding energies are almost twice as high than for (C2H4)Ag+ and still significantly higher than for (C2H4)Cu+. Structural features and energetics obtained at the various computational levels, although they differ significantly in their computational demands, are in satisfying agreement with each other, adding to the level of confidence that can be attributed to the computationally economic DFT and DFT/HF hybrid methods. In order to determine the nature of the bonding in these (C2H4)M+ complexes, an energy decomposition scheme is applied to the DFT results. For all three metal cations, the interaction with ethylene shows large covalent contributions. The major part of the covalent terms stems from σ-donor contribution from the ligand to the metal, whereas π-acceptor bonding (back-bonding) is less important. An atoms-in-molecules (AIM) analysis of the charge density distribution reveals cyclic structures for (C2H4)Au+ and (C2H4)Cu+, whereas (C2H4)Ag+ is T-shaped.

Nonrelativistic and quasirelativistic density-functional calculations have been performed aimed at investigating structures and bonding in the cationic methylene complexes MCHz+ of nickel, palladium, platinum, iridium, and gold. Relativistic effects on the bond dissociation energies of these complexes amount to 4-7, +15, +51, -3, and +66 kcal/mol, respectively. The influence of relativity on bond energies is analyzed in detail and discussed in terms of the s-orbital stabilization and d-orbital destabilization. The relativistic contraction of the M+-CH2 bond is found to be small for the group 10 metals and iridium but significant for gold.

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The equilibrium bond distances, harmonic frequencies, and bond dissociation energies of the 21 homonuclear diatomics Li,-F,, Na,-Cl,, and K,-Br, have been determined using approximate density functional theory (DFT) employing various widely used functionals and basis sets ranging from single zeta to triple zeta plus polarization quality. The results are in general much less sensitive to the size of the basis set as in conventional ab initio molecular orbital (MO) theory, while the choice of the functional is of much more significance. For one basis set (6-311G*), the performance of the DFT-based calculations has been compared and found to be superior to Hartree-Fock ( tional ab initio methods for the calculation of molecular properties like binding energies, equilibrium geometries, or harmonic frequencies has increased dramatically over the last decade.',' It is not the aim of this article to review developments in density functional theory, but to introduce the terminology and to enable the reader to place the remainder Of this article into a broader context, we

A D2h‐symmetric structure with a delocalized π electron system describes the ground state of s‐indacene (1), although it is a formally antiaromatic system. These results, obtained by both high‐level ab initio MO methods and calculations based on density functional theory, are in agreement with the known structure of 1,3,5,7‐tetra‐tert‐butyl‐s‐indacene.

The cationic gold(1) complexes Au+(HzO), Au+(CO), Au+(NH3), and Au+(CzH4) have been examined by different ab initio and density functional methods using nonrelativistic and relativistic ECPs and a quasi-relativistic approach, where relativistic effects are explicitly taken into account. On the one hand, Au+(HzO) and Au+(CO) exhibit binding energies (35.9and 44.1 kcaymol, respectively, a t the CCSD(T) level of theory) which are comparable with those of the complexes of the group 11 congeners Cu+ and Ag+. While for Au+(NH3) and Au+(C2H4) a large relativistic stabilization is observed, such that the binding energies (65.3 and 68.8 kcaymol, respectively, at the CCSD(T) level of theory) are almost twice as high as for M+(NH3) and M+(CzH4) for M = Cu and Ag. With respect to the computational methods applied, structural features and energetics obtained with different ab initio (MP2, CCSD-(TI) and DFT (ADF/BP, B3LYP) methods are in reasonable agreement with each other. In general, the relativistic effects on structures and energetics of these gold(1) compounds are quite large, and the interplay of electron correlation and relativistic effects is discussed.