Haptotropic Shifts in Cyclopentadienyl Organometallic Complexes: Ring Folding vs Ring Slippage

The coordination geometry of shifted cyclopentadienyl ligands (Cp = C 5 H 5 - ) in organometallic complexes resulting from a two-electron reduction of η 5 -Cp parent species was investigated by means of molecular orbital calculations performed with the B3LYP HF/DFT hybrid functional. Two types of complexes were studied: piano-stool mono(cyclopentadienyl) complexes, [(η 5 -Cp)M(CO) 3 ], and bent metallocenes, [(η 5 -Cp) 2 M(CO) 2 ] 3+ with first (M = Mn) and third (M = Re) transition row metals, to study the influence of the coordination geometry as well as the metal size on the cyclopentadienyl haptotropic shift. The electronic structure of those species and of the reduced complexes, [(η-Cp)M(CO) 3 ] 2 - and [(η-Cp)(η 5 -Cp)M(CO) 2 ] +, was analyzed with special emphasis on the (η-Cp)−M coordination. The reduction yielded haptotropic shifts in all cases, but the resulting (η-Cp)−M coordination geometry proved to depend on both the complex geometry and the metal size. For the bis(cyclopentadienyl) species folded η 3 -Cp compounds were obtained, with significant folding angles: ω = 13° (M = Mn) and 18° (M = Re), while the reduced mono(cyclopentadienyl) complexes present slipped planar Cp ligands. The enhanced stability of the folded η 3 -Cp geometry for the bis(cyclopentadienyl) complexes is related with the mixing of a metal z 2 type orbital into the reduced species highest occupied molecular orbital (HOMO), a Cp−M π* orbital that becomes occupied with the reduction.