The rare-gas rule for the electronic structure of metal carbonyls and other complexes is discussed in relation to changes in the electrostatic potential field of the metal caused by the ligands.In the octahedral (ds) and the tetrahedral (dl0) system the electrostatic potential of the carbonyl ligan ds is very nearly spherically symmetrical, up to 1-25 and 1-0 a.u., respectively, from the metal nucleus. It is in this region that the self-consistent field d-orbitals have a maximum charge density.If electrons are transferred from a first-series transition metal to carbonyl groups in x-donor bonding, the reduced screening has the effect that the averaged potential approaches that of krypton in radial dependence as well as in spherical symmetry, in contrast to the situation for monoatomic ligands such as F-. Support is thus found for the view that, when the o-bonds are formed by ligand lone-pair donation, the rare-gas rule will apply only if there is substantial electron transfer from the metal x-orbitals. Expansion of d-orbitals under the influence of ligand lone pairs is probably important in facilitating electron-withdrawal.THE " rare-gas rule " expresses a feature of the electronic structures of a large number of complexes of transition metals, especially in low valency states. The rule asserts that the number of electrons from the metal added to the number of bonding electrons from the ligands equals the number of electrons in a rare gas. In complexes of first-row transition
A theory of electronic resonances based on spin-coupled wavefunctions is presented. The formulation is state selective and yields the best single-configurational minimax wavefunctions for resonance closed channels. The theory is demonstrated by calculations of positions, shifts and widths of resonances of a test system, s-wave helium.
W.2Mulliken's method for obtaining gross atom-charges has been modified by use of a partitioning of the overlap densities which preserves their dipole moment. The gross atom charges are now determined by the elements of (Pg), rather than (PS), where 5 is a non-symmetric matrix whose elements depend upon the various overlap integrals, internuclear distances, and centroids of the overlap densities. The method is applied to the ground-states of some first-row diatomic molecules. For comparison, contours of the one-electron density function, and a difference density function, are presented for BH and FH. The effective atomic charges and contour diagrams are both calculated by use of two different sets of atomic basis functions. The calculated effective atomic charges are sensitive to both the choice of basis functions and the method used for partitioning the overlap densities.THE population analysis of LCAO molecular orbital wavefunctions was originally proposed by Mulliken to provide a means of extracting useful chemical information from calculated molecular wave-functions. Mulliken also described the calculation of gross atomcharges, or effective atomic charges, from the elements of the atom bond-population matrix, P, which is constructed from the atomic-orbital coefficients of the occupied molecular-orbitals. Following this work, it has now become widespread practice, in both semi-empirical and ab initio calculations, to derive a set of effective atomic-charges from the calculated molecular-orbital wave-function. These effective atomic-charges are then frequently used to discuss the extent of charge transfer within molecules, but there have been few attempts to This paper presents the results of a population analysis on the ground-states of some first-row diatomic molecules : two methods for estimating the effective atomic charges were used. The results for BH and FH are compared with the corresponding density-functions, P( 1,l). Finally, difference density-functions are plotted for these molecules to show more clearly how the electron distribution changes on molecule formation.
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