Synthesis and structure of bivalent ytterbocenes and their coordination chemistry with pi-acceptor ligands

The bivalent lanthanide metallocenes [1,3-(Me3C)2CsH 3]2Yb and (Me4C5H)2Yb have been synthesized and their structures have been determined by X-ray crystallography. Comparison with the known structures of (Me5C5) 2Yb and [1,3-(Me3Si)2C5H3] 2Yb leads to an understanding of the role of intermolecular contacts in stabilizing these coordinatively unsaturated molecules. The optical spectra of the base-free ytterbocenes and their Lewis-base adducts have been measured; the position of the HOMO - LUMO transition can be correlated with the degree of bending of the complexes in solution according to a molecular orbital model. Electron - electron repulsion, resulting from additional sigma-donor ligands, also affects the HOMO - LUMO transition by increasing the energy of the filled f-orbitals.; The base-free metallocene (Me5C5)2Yb coordinates carbon monoxide, resulting in a decrease in nuCO relative to that of free carbon monoxide. This behavior is reminiscent of d-transition metallocene chemistry. Other base-free ytterbocenes also coordinate carbon monoxide and the degree of back-donation is related to the substituents on the cyclopentadienide rings. Isocyanides are coordinated in a 1:2 ratio by the ytterbocenes, giving complexes having nuCN higher than those of the free isocyanides. An electrostatic bonding model has been used to explain the changes in CN stretching frequencies. The optical spectra of the carbonyl and isocyanide complexes are consistent with the molecular orbital model of the variation in the HOMO - LUMO gap upon bending, and the increase in electron - electron repulsion due to the additional ligands.; The complex (Me5C5)2Yb(bipy) exhibits optical, infrared and NMR spectroscopy and an X-ray crystal structure consistent with the presence of bipyridyl radical anion. However, the shape of the magnetic susceptibility curve is unusual. 2,2'-Bipyridyl and 1,10-phenanthroline complexes of several ytterbocenes have been prepared for comparison with this molecule. The phenanthroline complexes display strong electron exchange coupling, with the degree of coupling dependent on the substituents on the cyclopentadienide rings. The magnetism of the bipyridyl complexes may also be ascribed to electron exchange coupling, or to an equilibrium between a diamagnetic bivalent ytterbium complex and a trivalent complex resulting from the transfer of one electron from the metal to the bipyridyl ligand.