Abstract
In the course of investigating 7/2-bound arene complexes of
pentaammineosmium(II),1 we have consistently observed the
formationoftraceamountsofa speciesinwhichthearene appears
to act as a bridging ligand. Recently we reported that [Os-
(NH3)5(i|2-C6H6)](OTf)2 (OTf= CF3S03') (1), when allowed
to stand in an inert solvent for24 h, condenses to form the bi-
nuclearspecies [(Os(NH3)5)2(j72:j/2-M-C6H6))](OTf)4 (2) andfree
benzene in high yield.2 'Hand 13C NMR data indicate three
inequivalentpositionsinthering,andchemical shiftdatasuggest
thatthemetalcentersengageseparatedoublebonds. Thisbonding
arrangementwould resultina single localizedolefinsitethatis
expected toshow enhanced reactivityrelativeto the free ligand.
However, whereas single-metalcoordinationhas been shown to
activatean arene towardhydrogenation3or furthercoordination,4
double-metal coordination appears to limit further reactivity:
attempts to add a third pentaammineosmium(II) moiety were
unsuccessful,as were our attemptsto hydrogenate3or epoxidize5
the complex. We attributed the failure of2 to undergo these
reactions to stericratherthan electroniceffects, reasoning that
stericconstraints requirethemetalcenterstolieon oppositefaces
ofthearene plane. Tosettlethequestionofgeometry, we have
determined the crystal structure for 2....
