Theoretical Studies on the structure and bonding of metallacyclocumulenes, - cyclopentynes, and - cycloallenes

Abstract

In this paper, we compare the electronic structure of the hafnacycloallene complex Cp₂HfC₄RR'₂R'' (5Hf), which was previously described by Erker et al., with those of the titanium, zirconium, and hafnium complexes Cp₂M(η⁴-RHC₄HR) (3M; i.e. metallacyclopent-2,3,4-trienes, metallacyclocumulenes) and Cp₂M(η⁴-R₂C₄R₂) (4M; i.e. 1-metallacyclopent-3-ynes) using density functional theory (BP86/LANL2DZ) calculations. Moreover, the η³-phenylallenyl zirconocene complex 7Zr, which was synthesized by Wojcicki et al., is included for the comparison. These calculations and extended Hückel calculations show that the bonding in complex 5Hf is remarkably similar to that of complexes 4M and 7Zr. An analysis of the structural parameters and bonding reveals that the unique interaction of the internal carbon atoms along with the terminal carbon atoms with the bent-metallocene moiety is the reason for the unusual stability of these metallacycles. The molecular orbital analysis further suggests that complex 5Hf can react with another metal fragment to give the bimetallic complexes 9 and 10. The electronic structures of complexes 3M, 4M, 5Hf, and 7Zr have been comparatively studied to get a general understanding of the bonding in these metallacycles.