Haptotropic shifts in organometallic complexes with η 5-coordinated π ligands

Abstract

The electronic structure and bonding of a series of tricarbonylmanganese complexes, [(η n -X)Mn(CO) 3] m , were studied by means of ab initio calculations using the B3LYP hybrid functional with a LANL2DZ basis set. The following complexes with hydrocarbon ligands were addressed ( n=5, m=0): X=cyclopentadienyl (Cp=C 5H 5 −), indenyl (Ind=C 9H 7 −), fluorenyl (flu=C 13H 9 −), cyclopenta[ def]phenanthrenyl (cpp=C 15H 9 −), 8-hydro- as-indacene ( as-ind=C 12H 9 −) and 5-hydro- s-indacene ( s-ind=C 12H 9 −). Two complexes with heterocyclic ligands, X=pyrrolyl (pyr=NC 4H 4 −, n=5, m=0) and X=thiophene (tp=SC 4H 4, n=5, m=1+) were also studied. For all those species, in which X is a η 5-coordinated π ligand, the bonding was analysed, and the optimised structures were compared with the experimentally determined X-ray structures, available from the literature. The results show a weakening of the (η 5-X)Mn bond with the increasing of the ligands π system extension. The η 3 species resulting from a two electrons reduction of the previous complexes were studied, their structures optimised, and a comparison between the different coordination geometries of η 3-X is provided. Thus, for the fluorenyl and cyclopenta[ def]phenanthrenyl complexes, an exocyclic allylic coordination was found for the reduced species ( n=3, m=2−). For the indenyl and indacene reduced complexes two possible coordination geometries were studied, the exocyclic and the folded η 3-X, the latter being found the more stable for all these species (X=Ind, as-ind, s-ind; n=3, m=2−). The heterocyclic ligands were shown to adopt a σ rather than a η 3 coordination geometry, in the corresponding reduced complexes ( m=2− for pyr and m=1− for tp).