Chemical applications of topology and group theory: 37. Pentalene as a ligand in transition metal sandwich complexes

The η 8 ‐C 8 H 6 2− pentalene ligands in the recently discovered sandwich compounds (η 8 ‐C 8 H 6 ) 2 M (M = Ti, Zr) can be derived either by transannular ‘pinching’ of the cyclooctatetraene dianion, C 8 H 8 2− , or by edge‐sharing fusion of two cyclopentadienyl ligands C 5 H 5 − . This paper depicts the relationship between the molecular orbitals in C 8 H 6 2− , C 8 H 8 2− , and 2C 5 H 5 − . In addition, this paper shows that the d   z   2orbital of the central metal atom is of suitable symmetry for δ bonding in the D 2h complexes (η 8 ‐C 8 H 6 ) 2 M (M = Ti, Zr) unlike in the D 8h complexes (η 8 ‐C 8 H 8 ) 2 M derived from cyclooctatetraene. This can account for the fact that the cyclooctatetraene sandwich compounds of stoichiometry (C 8 H 8 ) 2 M have unsymmetrical (η 8 ‐C 8 H 8 )(η 4 ‐C 8 H 8 )M structures in the case of the d‐block metals titanium and zirconium in contrast to perhapto structures (η 8 ‐C 8 H 8 ) 2 An in the case of the actinides (An = Th, Pa, U, Np, Pu) where metal f orbitals are available for metal–ring bonding. Copyright © 2003 John Wiley & Sons, Ltd.