Diosmium

Of the Group 8 metals, OSj complexes are the least well developed. In this section, we summarize the small family of Osj complexes, most of which have been reviewed previously [39,137]. A majority of Osj complexes are paddlewheels or have tetragonal geometries, akin to their Ruj counterparts. They also occur in similar oxidation states, namely OSj +, OSj +, and Os . However, a key difference is that the Osj core is the most prevalent (stability of Osj OS2 ), whereas the lower oxidation state of RUj is more common (stability of RUj + RUj +). Finally, an OSj complex has been isolated in the unusually high oxidation state of +7, which remains unknown for Ruj and Fej cores, with the exception of the (Ruj +N ) core in the diruthenium nitride complex (vide supra) [113,115]. 

Formamidinate Similar to aminopyridinates, formamidinate ligands stabilize OSj paddlewheels. The Os2(p-ArNCHNAr)4Cl2 (where Ar=/ -tolyl) was crystallized, and an X-ray diffraction study revealed a long Os-Os bond distance of 2.4672(6) A (FSR 0.98) [158]. The CV showed a quasi-reversible reduction at —0.62 V (vs FeCp2 ) and an irreversible oxidation at 0.60 V. The temperature dependence of the magnetic moment was simulated by adopting S= 1, = 1.802, 

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Reaction of OsCl6 with acetic acid/acetic anhydride mixtures containing concentrated HC1 gives the diosmium compound Os2(OAc)4C12 (rather than mixed-valence species, see section 1.8.3) other carboxylates can be made by carboxylate exchange ... 

The chemistry of osmium has been reviewed in COMC (1982) and COMC (1995)339 370 as well as in Comprehensive Coordination Chemistry II.340 A recent review covers the applications of diosmium tetracarboxylates.341... 

The salt Rb3[Os2Brg] was the first structurally confirmed example of a diosmium confacial... 

Unlike the remarkably stable mixed-valence diosmium complex, the much less stable heterodinuclear [(NH3)5Os(N2)Ru(NH3)5]5+ decomposes to [Os(NH3)5(N2)]2+ and [Ru(NH3)5(OH2)]3+ in water over a period of hours (537). This is to be contrasted with the Ru2 mixed-valence analog that decomposes over a matter of minutes at room temperature (536). The heterodinuclear +5 and +4 ions have been reported by a number of workers (538-540), though not completely characterized until recently (537). Similarly, the oxidation of the +5 ion to the +6 ion has been known for some time (539). However, its properties have only recently been studied in solution by Ce(IV) oxidation, or in the solid state, by the Cl2 oxidation. It is much less stable than its diosmium analogs. There is also evidence for the formation of the unstable [ (NH3)5Os(N2)Rh(NH3)5]5+ heterodinuclear species from the reaction of [Os(NH3)5(N2)]2+ with [Rh(NH3)5(0S02CF3)]2+ in neat CF3S03H (537). 

Over the past few years there has been a growing body of literature on the photophysics and photochemistry of organometallic Os complexes. A significant portion of the literature has focused on the photochemistry and transient photophysical characterization of triosmium clusters involving Os - Os bond homolysis. There is also a significant body of literature on CO loss from diosmium and triosmium complexes. 

R. P. Tooze, M. Motevalli, M. B. Hursthouse, and G. Wilkinson, Alkyl Compounds of Diruthenium(III) and Diosmium(III). X-Ray Crystal Structure of the First Ruthenium Peralkyl, Hexakis(neopentyl)diruthenium(III), J. Chem. Soc., Chem. Comm. 1984,799-800. 

Diosmium(II,II) Compounds. The only known examples of these are porphyrin compounds, Os2(P)2, that are quite comparable to their ruthenium analogs123. They are prepared in a similar way, i.e., by pyrolysis of Os(P)py2 compounds. Again, mixed ligand dimers can be obtained, such as (TTP)Os=Os(OEP), and in addition the mixed metal compound, (OEP)Ru=Os(OEP) has also been formed and identified by NMR, although not isolated. No structural data have yet been reported for an Os2(P)2 compound. 

The diruthenium and diosmium species M2(SiMe3)2(CO)g are obtainable in high yield and are reduced by sodium amalgam to [M(SiMe3XCO)4] , whose reactions with various halides provide a route to silicon-metal complexes - . Addition of silicon halides to [M(CO)4] " is successful only in the synthesis of Fe(SiH3)2(CO)4 (see 5.8.3.3.1.) and is not a route generally available for M(SiR3)2(CO)4. 

Redox properties of all the complexes except 7 were studied in acetonitrile. The complex 7 was studied in aqueous phosphate buffer solution (pH, 6.88). While the complexes 1 - 3 are isolated in trivalent (III,III) states, and 4 in tetravalent (IV,IV) and one-electron reduced mixed-valence (in,rV) states, their cyclic voltammograms all display two one-electron reversible redox processes between the oxidation states (III,III) and (IV,IV) in acetonitrile. The mixed-valence diosmium(in,IV) complex has been isolated and its stmeture has been determined by the X-ray stmctural analysis. Three dinudear Ru(ni) complexes are reduced with a quasi-reversible one-electron and irreversible one-electron processes. The oxo-centered trinuclear mthenium(in) complex displays four one-electron reversible processes om (II,II,III) to (in,IV,IV). Complexes with three different oxidation states (II,III,III),... 

II,III,III) state of the oxo-centered trinuclear ruthenium complex 9 and the (III,IV) state of the oxo-bridged diosmium complex 4 is noted. 

A similar diosmium complex was found by Churchill s group (Samkoff et al., 1984). 

Introduction and survey/Multiple bonds in dirhenium and ditechnetium compounds/ Multiple bonds in dimolybdenum and ditungsten compounds/Multiple bonding between chromium atoms/X3M=MX3 compounds of molybdenum and timgsten/ Diruthenium and diosmium compounds/Dirhodium compounds/Dimetal com-... 

Tubocurarine complexes. Reaction of Os(bipy)(Cl4 with (—)-tubocurarine (L) gave the binuclear complex [Cl(bipy)Os(LL)Os(LL)Os(bipy)Cl], dichloro-/i-[l,4-cyclohexylbis(2,2, 2"-terpyridyl)-6"-methyl)-l,4-diol]bis(2,2 -pyridyl)diosmium(II) dichloride. The two coordinated chloro ligands can be replaced by pyridine on refluxing this with the complex in aqueous solution. The former complex has potent curariform biological activity and there are possible applications as stains for electron microscopy for the complexes. ... 

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