Most stable organic and organometallic molecules are diamagnetic with a closed-shell configuration, such that electron pairing is often considered as one of the fundamental rules of molecular chemistry. For the great majority of molecular chemists, magnetism is a far-off science which has nothing to do with their own field of research. The situation is obviously different for both coordination chemists working with transition metal ions and organic chemists interested in free radicals. The entities they are dealing with may have an open-shell configuration. However, the interactions

The electronic structure of the prototypical unsaturated and saturated organic polymers polyacetylene and polyethylene is elucidated by a building-up process starting from a linear carbon chain, which is then kinked and finally has one or two hydrogen atoms attached to it. In this process the common features as well as the differences between polyacetylene and polyethylene (semiconductor vs large band gap insulator, unsaturation vs saturation) emerge in a natural way. The emphasis is on bringing together the essential concepts of solid-state physics and simple ideas of chemical bonding. Some topics of special interest, such as bond alternation and solitons in polyacetylene, are also discussed, in as simple and chemical manner as possible.

Starting from a configuration interaction (CI) approach including only doubly excited configurations, the corresponding energy functional is modified by introduction of a topological factor in the normalization condition of the CI coefficients in such a way that it gets inherently size extensive. Constraints imposed by N representability conditions on the modified functional are discussed and lead to a specific choice of the topological factor. The basic variable in the modified energy functional is the second-order reduced density matrix parametrized in terms of CI-like coefficients. Test calculations for a variety of small molecules show that the numerical results obtained with the new functional are in very good agreement with those obtained from coupled cluster singles doubles calculations.

A correlation between the orientation of [TCNE] •-(TCNE ) tetracyanoethylene) bound to a [Mn III -(por)] + (por ) substituted meso-tetraphenylporphyrin) and the magnitude of magnetic coupling for a series of previously prepared [Mn(por)][TCNE]‚2PhMe linear-chain molecule-based ferrimagnets has been identified. The tetraphenylporphyrin (H 2 TPP), tetrakis(4-chlorophenyl)porphyrin (H 2 TClPP), tetrakis(4-methoxyphenyl)-porphyrin (H 2 TOMePP), tetrakis(2-fluorophenyl)porphyrin (H 2 TFPP), and tetrakis(3,5-di-tert-butyl-4-hydroxyphenyl)porphyrin (H 2 TP′P) ligands have all been utilized. Previous structural determinations as the ditoluene solvates indicate that the Mn-N distance varies ∼2.6%, while the Mn-(N-C) TCNE bond angle varies by as much as 25.2% from the mean values over this series of compounds. Hence the overlap between the [TCNE] •-π* SOMO and the Mn III SOMO d orbitals plays a significant role in controlling the magnetic properties. From molecular orbital overlap considerations, as determined from semiempirical INDO/SCF calculations, the expected d π -π* (d π ) d xz , d yz ) overlap is not as important as the σ-d z 2 -π* overlap between Mn III and the [TCNE] •-. Furthermore, the greater the deviation from 90°for the dihedral angle between the mean MnN 4 [Mn(por)] + plane and the [TCNE] •-mean plane increases the σ-d z 2 /p z overlap between Mn III and the [TCNE] •-leading to an increased intrachain coupling as the angle is decreased. An increase in the T min (the temperature at which the minima in the temperature dependence of the moment occurs) is observed as this angle becomes more acute, reflecting the stronger magnetic coupling. This suggests systems with smaller Mn(por) + /[TCNE] •-dihedral and Mn-(N-C) TCNE bond angles should have enhanced intrachain magnetic coupling, J intra , leading to higher T c 's. Pressure may force such systems to have reduced angles and also lead to higher T c 's.