Publications

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....

When 0s(NH3)3(CF3S03)3 is reduced in the presence of diphenylacetylene, four pentaammineosmium(II) complexes can be isolated

and characterized in which the alkyne remains intact. The kinetically favored mononuclear complex features a phenyl group

i;2-bound to the metal center. This species is observed to undergo a linkage isomerization in which the osmium migrates to the

alkyne. Through the use of pulse radiolysis, this isomerization was determined to be greatly accelerated by oxidizing the metal

to Os(III). The inherent stability of alkyne complexes for both di- and trivalent osmium provides the opportunity to assess the

ability of this ligand to act as a -donor as well as -acceptor.

The hydration of unactivated alkynes represents an important

method of functionalizing this plentiful hydrocarbon resource and

has found considerable synthetic use.1 Transition metals are

widely used to catalyze this process as well as the analogous

reaction in which alcohols are added across the triple bond.2

Though -vinyl ether3 and -vinyl alcohol3,4 complexes are un-

doubtedly intermediates in these reactions, to our knowledge there

have been no reports of such species resulting from an ^-coor-

dinated alkyne. In an early paper on the reactivity of ?;2-alkyne

complexes of platinum(II), Chisholm and Clark suggested that

addition of methanol occurred across the alkyne bond to produce

a vinyl ether intermediate, but this suggestion was later with-

drawn.5 Here we report that the alkyne complex [Os(NH3)5-

(p2-CH3CCCH3)]2+ reacts quantitatively with methanol or water

to form -vinyl ether and -vinyl alcohol complexes, respectively.

Reduction of the precursor Os(NH3)5(OTf)3 (OTf= CF3S03")

in the presence of 2-butyne results in a complex, 1, which is readily

characterized as [Os(NH3)5(p2-CH3CCCH3)](OTf)2.6 Though

the thermal instability of this material has precluded a successful

microanalysis,7 convincing evidence for this assignment is provided.....

Abstract: Reduction of (NH3)5Os(TFMS)3 (TFMS= trifluoromethanesulfonate) by Co(Cp)2 in the presence of cationic pyridines

L (L= jV-methylpyridinium, lutidinium, pyridinium, and A-methyl-4-picolinium) leads to the formation of isolable penta-

ammineosmium(II) complexes featuring fluxional 3,4-t;2 ligands for the former three and 2,3-t;2 for the latter. Analogously

to the previously reported i)2-lutidine analogue, activation at the C4-H bond was observed for the 3,4-rj2-bound cationic ligands,

yielding carbon-bound pyridinium ylides. In accord with observations made on other complexes containing metal-ylide carbon

bonds, the resulting pentaammineosmium(II) /V-methylpyridinium complex is unstable with respect to loss of the trans ammine.