THREE IS A CROWD. A GEOMETRICAL ANALYSIS OF (COD)RH/IR COMPLEXES WITH TRIDENTATE N-DONOR LIGANDS.

RJ.N.A.M. Kicken, P.H.M. Budzelaar, R. de Gelder, J.M.M. Smits, and A.W. Gal.

Department of Inorganic Chemistry, University of Nijmegen, Faculty of Sciences, Toernooiveld I, 6525 ED Nijmegen, The Netherlands

NMR data have shown that the energy barrier for the rotation of 1,5-cyclo-octadiene is quite low in five coordinate complexes of Rh(I) and Ir(I) with tridentate N-donor ligands. Ab initio calculations indicate that the energy differences between all possible conformers are fairly low (max. 7 kcal/mole).

Figure l. Both the trigonal bipyramidal (1) and the square pyramidal form (2) of the cationic compound [BMIMARRh(cod)]⁺ are found in one unit cell. Indicating that the energy differences are low and can be overcome by packing forces.

Subtle changes in tridentate nitrogen donor ligands have provided us with a large number of similar compounds, the structures of which have been analyzed by X-ray diffraction. We find a range of easily accessible coordination geometries extending from square pyramidal to trigonal bipyramidal structures. Analysis of bond lengths and angles suggests that the latter type is best interpreted as a square planar structure with two N- donor atoms occupying a single coordination site.