Osmium atom-transfer reactions

Atomic volume, of actinide metals, 31 36 Atom-transfer reactions, osmium, 37 340-... 

Table 2 Representative kinetic data for the nitrogen atom transfer reactions by osmium(VI) nitrido...

E. Reactions of Ligands on Osmium 1. Atom-Transfer Reactions... 

This observation may well explain the considerable difference between metal-olefin and metal-acetylene chemistry observed for the trinuclear metal carbonyl compounds of this group. As with iron, ruthenium and osmium have an extensive and rich chemistry, with acetylenic complexes involving in many instances polymerization reactions, and, as noted above for both ruthenium and osmium trinuclear carbonyl derivatives, olefin addition normally occurs with interaction at one olefin center. The main metal-ligand framework is often the same for both acetylene and olefin adducts, and differs in that, for the olefin complexes, two metal-hydrogen bonds are formed by transfer of hydrogen from the olefin. The steric requirements of these two edgebridging hydrogen atoms appear to be considerable and may reduce the tendency for the addition of the second olefin molecule to the metal cluster unit and hence restrict the equivalent chemistry to that observed for the acetylene derivatives. 

Figure 10 Plot of rate constants for back electron transfer from Sn02 to electrostatically bound ruthenium ( ) and osmium ( ) complexes as a function of the number of carbon atoms comprising alkyl spacers. Within experimental error, the driving force for each series of reactions is unaffected by changing the size of the alkyl spacer.

As already indicated, the chemistry of terminal borylene complexes is as yet almost unexplored. In addition to the photochemically induced borylene transfer, which was already discussed in Chapter 3.2, studies of the reactivity of terminal borylene complexes are restricted to two recent reports by Roper.147,148 The base-stabilized borylene complex [Os (=BNHC9H6N)Cl2(CO)(PPh3)2] (26) undergoes a reaction with ethanol to yield the ethoxy(amino)boryl complex [Os B(OEt)NHCgH6N Cl(CO) (PPh3)2] (35) according to Eq. (13) with a 1,2-shift of the quinoline nitrogen atom from the boron to the osmium center. The alcoholysis of 26 indicates that even the boron atom in base-stabilized borylene complexes displays some electrophilic character—a fact already predicted by a theoretical study.117... 

Osmium and ruthenium in high oxidation states (III, IV, or V) form polypyridine (bpy, phen or tpy) hydroxo and 0x0 complexes, which have a rich redox chemistry [166, 167]. Redox changes are metal-localized and accompanied by reactions of M=0 or M-OH bonds. These complexes are active as electrocatalysts of the oxidation of water to O2, or of CF to CI2, or they can transfer oxygen atoms and oxidize organic substrates like (CH3)2CHOH or C6H5-CH(CH3)2. 

Metals belonging to the second and third rows, which are used in several catalytic processes, have been intensively studied. Zr [62], Nb [63] and Ta [64] atoms show the formation of OM(CO) and 02M(C0)2 products. The process is barrierless for Zr, whereas Nb and Ta transfer electrons to CO2 and promote the formation of adducts as intermediates. Laser ablated Co and Rh atoms [65] show similar reactions with formation of neutral species such as OM(CO), 02M(C0) and OCo2(CO). Laser ablated Re, Ru and Os atoms in Ar and Ne matrices [66, 67] afford divers species of the type OM(CO), 02M(C0) and 020s(C0)2 (formed by reaction of OOs(CO) with a second CO2 molecule), and 0CRu(02)(C0) obtained by addition of a CO2 molecule to ORu(CO). Osmium is more reactive than ruthenium. 

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