摘要：

The mechanism of formation of mer,trans-[(PMe3)(3)Rh(-Cequivalent toC-R)(2)H] from [(PMe3)(4)Rh(Me)] and terminal alkyne has been studied. The initial step of the reaction is the elimination of methane and the formation of the trigonal bipyramidal complex [(PMe3)(4)Rh(-Cequivalent toC-R)], a reaction that is complete in time of mixing at -78 degreesC. This intermediate undergoes an oxidative addition reaction with a second equivalent of alkyne to give fac-[(PMe3)(3)Rh(-Cequivalent to C-R)(2)H] as the kinetic product. This fac isomer is not stable above -20 degreesC and isomerizes to the thermodynamic product mer,trans-[(PMe3)(3)Rh(-Cequivalent toC-R)(2)H]. fac-[(PMe3)(3)Rh(-Cequivalent toCR)(2)H] will exchange alkynyl groups with free alkyne, a reaction that has a lower energetic barrier than the isomerization to mer,trans-[(PMe3)(3)Rh(-Cequivalent toC-R)(2)H]. Density functional theory studies on all stages of the formation of mer,trans-[(PMe3)(3)Rh(-Cequivalent toC-R)(2)H] have been carried out and give ground state energies in line with those experimentally observed. Once formed, mer,trans-[(PMe3)(3)Rh(-Cequivalent toC-R)(2)H] is configurationally stable and not prone to scrambling, although it will react with chloroform, whereupon the hydride is replaced by chloride. The initial product of this reaction is mer, trans-[(PMe3)(3)Rh(-Cequivalent toC-R)(2)Cl], and this compound has been studied by single-crystal X-ray diffraction. In solution mer,trans-[(PMe3)(3)Rh(-Cequivalent toC-R)(2)Cl] isomerizes slowly to mer,cis-[(PMe3)(3)Rh(-Cequivalent toC-R)(2)Cl]. [References: 69]

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