Fundamental aspects of Brønsted acidity in ionic liquid systems, in relation to those of simple protic molecules in the gas phase, pure protic molecules in the condensed phase and solutions of protic molecules in molecular systems, are presented. The variety of acidities possible, beyond those observed in aqueous systems, is emphasised and discussed in terms of differences of solvent levelling, ionisation, dissociation, homo-/hetero-conjugate ion speciation and the stabilisation of proton-transfer products from solvent to solvent. It is argued that data regarding aqueous systems do not necessarily explain acid/base behaviour in other liquids satisfactorily. Methods of measuring acidity are reviewed, particularly by spectrophotometry and electrochemistry and recommendations proffered for estimating speciation and acidity of ionic liquids of various complexities.

Die neuen Komplexe (Ia) wurden durch Reaktion von Co2(CO)8 mit Dimethylgerman oder ‐stannan dargestellt.

The six-coordinate W0 complex cis-W(CO)4(C5H5N)2 has been found to be active in the in situ formation of a carbene species from norbornene, which generates a typical ring-opening metathesis product (ROMP). A proposed mechanism of initiation suggests that the reaction involves a 2,3-hydrogen shift in the coordinated norbornene (η2 → η1). The initiating carbenoid group is identified from the products of the spontaneous carbene–CO coupling and Wittig reactions test. Formation of W(CO)3(η6-C6H5CH3) when toluene is the solvent, followed by reaction with the carbene, is blamed for catalyst deactivation.Key words: bis-pyridine-tetracarbonyl-tungsten, ROMP, olefin metathesis, norbornene.

Rearrangements of norbornadiene (NBD, C7H8) to various alkylidenes, via a hypothetical 7-coordinate tungsten(II) complex W(CO)3I2(NBD), were studied using density-functional theory computations. An extensive search for intermediates and transition states of rearrangement was made. The theoretical method (basis sets and level of DFT) used was justified by new benchmark studies which compare optimized structural parameters to those from crystal structures of several different tungsten complexes. Transition-metal-catalyzed rearrangements of NBD are not as well-known as those of norbornene and are considerably more complicated than had been thought. This work predicts a large variety of intermediates which may be feasible targets for experimental synthesis. All the rearrangement paths to alkylidenes found here feature high activation energies of over 45 kcal mol−1, implying that self-initiation for the ring-opening metathesis polymerization of NBD via tungsten(II) complexes must occur via an alternative mechanism.

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