Transits across a Cyclopentadienyl: Organic and Organometallic Haptotropic Shifts

Organic and organometallic haptotropic shifts in cyclopentadienyl compounds CpX are analyzed by studying the motion of an X+ across the face of a C5H5- ring. The ligand in transit, X+, may be H+, CH3+, XH3+ (X = Si, Ge, Sn), Mn(CO)3+, Li+, CUR+, and CH22+ and CH2, the latter serving as models for BR2+, AIR2+ and NR2+, PR2+ as well. The Cp- offers donor orbitals of a + e symmetry. The interactions of the ligand may be dominated by an acceptor orbital of symmetry type a (H+, CH3+) in which case 7' and q2 geometries are favored, or a set of acceptor orbitals of a + e symmetry (MII(CO)~+, Li+, CUR+), the e component of which greatly stabilizes the most symmetrical q5 coordination. The lower the energy of the e acceptor set and the better the overlap with its Cp counterpart, the more are q2 and q5 stabilized relative to 7'. In the case of XH3+ (X = C, Si, Ge, Sn) an e acceptor orbital moves to lower energy as one proceeds down the group, and this is responsible for the decreasing barrier for sigmatropic shifts in CpXH3. The XH2 case, which yields a number of interesting collapse struc- tures, is analyzed in detail for X, a main group center.