Two Types of σ-Allenyl Complexes from Reactions of Silylenes and Germylenes with Chromium Fischer Alkynyl(alkoxy)carbenes.

The reactivity of amidinatotetrylenes of the type E(tBu bzm)R (E=Si, Ge; tBu bzm=N,N'-bis(tertbutyl)benzamidinate; R =alkyl or aryl) with the chromium Fischer alkynylcarbene complexes [Cr{C(OEt)C R }(CO) ] (R =Ph; ferrocenyl, Fc) has been studied. At room temperature, two different reaction pathways have been identified: (a) attack of the amidinatotetrylene to the alkynyl C atom (γ-attack), which leads to σ-allenyl complexes in which the original C atom maintains its attachment to the Cr(CO) and OEt groups (compounds 3 ), and (b) attack of the amidinatotetrylene to the C atom (α-attack), which ends in σ-allenyl complexes in which the original C atom is not attached to the metal atom and has been inserted into an E-N bond of the amidinatotetrylene forming an E-C-N-C-N five-membered ring (compounds 4 ). It has been found that compounds 3 are thermodynamically less stable than their corresponding 4 isomers and that some of the former (E=Ge; R =CH SiMe ) can be transformed into the latter upon heating. At high temperatures (>70 °C) the reactions involving bulky amidinatotetrylenes (R =Mes, tBu) end in the carbene-substitution products [Cr{E(tBu bzm)R }(CO) ].