Theoretical studies on π-complex formation of organocopper compounds with acetylene.: The origin of nucleophilicity of organocuprates

Complexes between MeCu and Me2Cu- with acetylene have been examined with the aid of HF, MP2, and B3LYP methods. The structure of the MeCu/acetyrene complex was not much affected by the choice of the theoretical methods, since the effects of electron-correlation are rather small in this complex. The structure of the Me2Cu-/acetylene complex on the other hand, depends strongly on the method, since electron correlation plays the key role in back donation from the copper 3d(xz) orbital to the acetylene pi* orbital. As a result, the B3LYP and the MP2 methods, which take electron correlation effects into account, afforded a cupriocyclopropene pi-complex as observed experimentally, while the HF method predicted an electrostatic complex between Me2Cu- and the acidic acetylenic proton. Molecular orbital analysis indicated that the high nucleophilicity of organocuprates such as Me2Cu- is due to the back donation from the copper 3d(xz) orbital, which necessitates the use of a theoretical method that appropriately evaluates the electron correlation effects. Similarly, a d-pi* Me2Au-/acetylene complex was also located by the B3LYP calculations. (C) 1999 Elsevier Science B.V. All rights reserved.