The rates of self-exchange for several systems of the class MCp2X+/Cp2, where M is Ru or Os and X is Cl, Br, or I, have been measured by the NMR line-broadening method. For each metal, the rates increase along the halogen series as given above and are more rapid for = Ru than for the osmium analogues. In consideration of these data, the possibility is raised that these 2e self-exchange reactions and those of other metal ions take place by le steps.
Abstract. This paper presents a critical discussion of the perceived similarity between alkyl groups and mechanical gears. Specifically, we examine the notion that the availability of "free spaces" between the ligands of the central atom in an alkyl group leads to a reduction in the steric requirements of the group and, hence, to an effect on the rate of conformational interconversion. It is concluded that there is no compelling experimental evidence to support the operation of this type of dynamic gear effect.
STATIC AND DYNAMIC GEAR EFFECTSModels of alkyl groups or, in general, of n-fold chemical rotors bear a manifest resemblance to toothed gears. If the ligands bound to an alkyl group center (e.g. the hydrogen atoms in CH3 or the methyl groups in t-Bu) are likened to the teeth of a gear, a formal analogy can be drawn between the rotors and their mechanical counterparts. Under conditions of intramolecular crowding, arrangements of neighboring alkyl groups can then be compared to meshed gears, as is strikingly obvious merely by inspection of molecular models.' The meshing of alkyl groups in the ground state is one kind of gear effecr'" for which we propose the collective term static gear effect"The gear analogy has also been widely applied to the dynamics of conformational interconversions. We propose the term dynamic gear effect to describe the special effect on the rate or mechanism of a process which may be attributed to the intermeshing of a chemical rotor with a neighboring group. Based on this classical mechanical model, a variety of claims and suggestions have been advanced that the availability of "free spaces" between the ligands of the central atom in an alkyl group leads to a reduction in the effective size of the group and hence to an effect on rates of conformational interconversions. In this paper we present a critical discussion of some of these proposed dynamic gear effects following the classical approach, i.e. ignoring quantum mechanical phenomena (e.g. tunneling)," In order to place what is to follow into proper perspective, we devote the next section to a more detailed discussion of the gearing analogy.
THE NATURE OF DYNAMIC GEARINGThe central analogy, that between chemical and mechanical gears, is often invoked in context with the coupled motion of rotors: the torsion of "gear-meshed" chemical rotors plainly evokes an image of rotating cogwheels. This analogy rests on the formal similarity between meshed alkyl groups and external gears." In these chemical and mechanical systems, a disrotatory motion of the component parts would appear to be an obligatory feature." In actuality, profound dissimilarities exist between chemical and mechanical gear systems. One vital difference is that mechanical gears are designed to move with uniform angular velocity; this cannot be the case for chemical rotors. 11 Net disrotatory motion of chemical rotors by no means implies that the motions along the pathway connecting the initial and final states resemble those of mechanical cogwheels. Rotations of chemica...
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.