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Proton transfer and hydrogen bonding are two aspects of the chemistry of hydrogen that respectively govern the behaviour and structure of many molecules, both simple and complex, from water to DNA. The β-dicarbonyls exhibit both of these features, and in ways which have singled them out for detailed study for many years. They provide the best known examples of keto ⇆ enol tautomerism, with the advantage of slow proton transfer and high concentrations of the enol tautomers in most cases. These enols are stabilized by intramolecular OHO hydrogen bonds which at various times have been thought of as being centred, linear hydrogen bonds that are somehow incorporated into the delocalised π system to give "aromatic" systems. Research involving structural, spectroscopic and computational techniques has deepened our understanding of these compounds and changed our picture of them. The hydrogen bonding is surprisingly strong, surprising since it is neither centred, nor linear, nor involved in the ring's delocalized bonding, although it is certainly coupled to it. This review deals with controversies that have surrounded the β-dicarbonyls and discusses the current view that the enol tautomers are a double-minimum potential well with a low energy barrier. The review ends with a brief look at the β-thioxoketones, which provide an analogous system based on a heteronuclear SHO hydrogen bond in the enol or enethiol tautomer. [ABSTRACT FROM AUTHOR]

The Angular Overlap Model is applied to the calculation of the Jahn-Teller coupling constants for all possible ground states of ML6dx systems in Oh symmetry. In the weak field scheme the LSMLMS〉 basis is treated by an operator equivalent technique and results are also derived for the LSJMJ〉 basis to include the effects of spin-orbit coupling. In the strong field basis both high- and low-spin ground states are considered where these arise and in all cases the extent of quenching of Jahn-Teller activity due to spin-orbit coupling is evaluated. The derivation of the required angular overlap parameters and the general utility of the method are briefly considered. [ABSTRACT FROM AUTHOR]

As a consequence of the non-spherical symmetry of the copper(II) ion, d9 configuration, and of the influence of the Jahn-Teller, and pseudo Jahn-Teller effect on six-coordinate geometries, the stereochemistries of the copper(II) ion are characterized by non-rigid geometries (fluxional behaviour), and ranges of distorted geometries (Plasticity Effect). The latter may be connected by a series of Structural Pathways, which may be characterised by an ‘Electronic Criterion of Stereochemistry' for a related series of complexes, e.g. the [Cu(bipy)2X] [Y] complexes. [ABSTRACT FROM AUTHOR]

Metal Complexes of the allyl (C3H5) and cyclopentadienyl (C5H5) ligands have played a major role in the development of organometallic chemistry. Metal allyl complexes are well known for the many intricate organic transformations which they may mediate, yet often possess limited stability. Metal cyclopentadienyl complexes on the other hand possess very high stabilities but only very limited catalytic chemistry. The pentadienyl ligand (C5H7 as well as various methylated derivatives) has similarities to both the allyl and cyclopentadienyl ligands, and it has now been demonstrated that metal pentadienyl complexes possess not only thermal stability but also chemical and catalytic reactivities. This favorable combination then will allow for detailed correlations to be made between chemical reactivities and the physical natures (structural, spectroscopic, etc.) of these compounds. In this article the presently available information on metal-pentadienyl compounds is discussed in order to gain some understanding of their nature compared to the related allyl and cyclopentadienyl systems, and to anticipate future applications of metal-pentadienyls. Particular emphasis is given to the bis(pentadienyl)metal complexes, which have been referred to as the "open metallocenes". [ABSTRACT FROM AUTHOR]