Acetylide ligands, geometry

The chemistry of metal complexes featuring alkyne and alkynyl (acetylide) ligands has been an area of immense interest for decades. Even the simplest examples of these, the mononuclear metal acetylide complexes L MC=CR, are now so numerous and the extent of their reaction chemistry is so diverse as to defy efforts at a comprehensive review. " The utility of these complexes is well documented. Some metal alkynyl complexes have been used as intermediates in preparative organic chemistry and together with derived polymeric materials, many have useful physical properties including liquid crystallinity and nonlinear optical behaviour. The structural properties of the M—C=C moiety have been used in the construction of remarkable supramolecular architectures based upon squares, boxes, and other geometries. ... 

The adoption of two different geometries for these isoelectronic Au6Pt clusters is probably due to electronic differences between the carbon monoxide and acetylide ligands, as neither ligand is sterically demanding in these compounds. However, both cluster compounds adopt metal cage geometries that are best described as hemispherical, as expected from their valence electron counts. 

Electron density calculations have been carried out on Gp2ScR (R=H, Me, Et, Prn, vinyl, acetylide). Geometry optimizations revealed an agostic interaction for R= Et, whereas Me and Pr are bonded to the metal center without agostic interactions. Other density functional studies have been carried out on the complexes Cp2LnCl and Cp2LnX(THF) (Ln = La-Lu X = F, Cl, Br, I). In these mixed ligand complexes, Ln-Cp and Ln-THF bonds are... 

In another DPT study, it was found that the incorporatiOTi of a second copper atom into the model resulted in the calculated reactivity of such dinuclear alkynylcopper(l) complexes being dramatically increased as compared to the calculated reactivity of mmionuclear alkynylcopper(l) complexes [97], Calculations were also carried out to locate the positions of copper atoms in dinuclear copper(l) acetylides, transition states, and a metallacycle for a copper(l)-catalysed reactimi of propyne with methyl azide two copper atoms were located, one of which carried as a spectator ligand, for example a prop-l-ynyl anion. A calculated activation energy of 54.0 kJ mol was found for the foimatimi of a metallacycle in a reaction of propyne with methyl azide. The diagrams in Scheme 14 show the calculated geometries and distances separating the two copper atoms in a dinuclear copper (1) acetylide in structure A, in a calculated transition state in structure B, and in a cuprated metallacycle in structure C. 

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