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(tBu2bzm)R1 (E=Si, Ge; tBu2bzm=N,N′‐bis(tertbutyl)benzamidinate; R1=alkyl or aryl) with the chromium Fischer alkynylcarbene complexes [Cr{C(OEt)C2R2}(CO)5] (R2=Ph; ferrocenyl, Fc) has been studied. At room temperature, two different reaction pathways have been identified: (a) attack of the amidinatotetrylene to the alkynyl C2 atom (γ‐attack), which leads to σ‐allenyl complexes in which the original Ccarbene atom maintains its attachment to the Cr(CO)5 and OEt groups (compounds 3E-R1-R2 ), and (b) attack of the amidinatotetrylene to the Ccarbene atom (α‐attack), which ends in σ‐allenyl complexes in which the original Ccarbene 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 4E-R1-R2 ). It has been found that compounds 3E-R1-R2 are thermodynamically less stable than their corresponding 4E-R1-R2 isomers and that some of the former (E=Ge; R1=CH2SiMe3) can be transformed into the latter upon heating. At high temperatures (>70 °C) the reactions involving bulky amidinatotetrylenes (R1=Mes, tBu) end in the carbene‐substitution products [Cr{E(tBu2bzm)R1}(CO)5]. Amazing isomerization: Heavier tetrylenes A (E=Si, Ge) can attack the Cγ and Cα atoms of the chromium Fischer alkynylcarbene carbenes B to give the zwitterionic σ‐allenyl derivatives C and D. For E=Ge (but not for E=Si), the γ‐addition can be reversed at 50 °C, enabling the isomerization of CGe into the thermodynamically more stable complex DGe.