SOLUTION STRUCTURES AND DYNAMICS OF M(3)(CO)(12-X)(NCCH3)(X) (M=RU, X=1, 2, 3, M=OS, X=1, 2)

The solution structures and the dynamic behavior of the acetonitrile substituted cluster compounds M(3)(CO)(12-x)(NCCH3)(x) (M=Os, x=1 (1), x=2 (2); M=Ru, x=1 (3), x=2 (4), x=3 (5)) have been investigated by C-13 NMR spectroscopy. The acetonitrile ligands in 1-5 adopt axial positions. In the bisubstituted derivatives 2 and 4 the nitriles are bound to different metal atoms on opposite sides of the metal plane and in the trisubstituted compound 5 two of the three nitrile ligands lie on the same side of the metal plane. Like the parent metal carbonyls M(3)(CO)(12) (M=RU, Os) the osmium derivatives 1 and 2 and the ruthenium derivative 3 have ground state structures in which all CO ligands are terminally bound, whereas 4 and 5 display also bridging CO ligands. 1-5 are fluxional in solution and the fluxionality is dependent upon the degree of nitrile substitution. The low energy CO exchange processes are of the delocalized type following merry-go-round mechanisms (involving two or three metal centers) whereas localized exchange processes are found to be of higher energy. The solution structures and the fluxional behavior of 1-5 are in agreement with a general ability of the nitrile ligand(s) to stabilize bridging carbonyls.