Solution structures and dynamics of M 3(CO) 12− x(NCCH 3) x (MRu, x = 1, 2, 3; MOs, x = 1, 2)

Abstract

The solution structures and the dynamic behavior of the acetonitrile substituted cluster compounds M 3(CO) 12− x (NCCH 3) x MOs, x = 1 ( 1), x = 2 ( 2); MRu, x = 1 ( 3), x = 2 ( 4), x = 3 ( 5)) have been investigated by 13C 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.