Pentaammine Complexes of Osmium with pi-Acceptor Ligands.

The Activation of Aromatic Molecules with Pentaammineosmium(II)

“…In contrast, the pentaammineruthenium(II) complex of pyrazine has a p K b of 11.5, not far from the free ligand value. For the complex [Os(NH 3 ) 5 (SO 2 )] 2+ , the acidity of the bound SO 2 was found by Sen to be reduced by approximately 5 orders of magnitude relative to the free ligand (evaluated with respect to its conversion to SO 3 - in water) . For the ruthenium(II) analog, the effect is similar but not as dramatic, and ruthenium(III) coordination was found to increase the acidity of the SO 2 ligand …”

“…Although not central to the theme of this review, several other equilibrium measurements have been determined that further illustrate the degree to which pentaammineosmium(II) acts as a π base. Two of the most striking examples are found in the pentaammineosmium(II) complexes of pyrazine and SO 2 . Free pyrazine has a p K b of 13.6, but coordination by osmium(II) lowers the p K b to 6.6, indicating that pyrazine becomes 7 orders of magnitude more basic as a result of π back-bonding.…”

η2-SO2 Linkage Photoisomer of an Osmium Coordination Complex

“…8 One reported attempt to coordinate SO2 to osmium pentaammine produced an impure compound. 9 Interestingly, ruthenium SO2 tetraammine coordination chemistry is rich in solid-state linkage photo-isomerization; [10][11][12][13][14][15][16][17] and pentaammines are a special case of tetraammine with NH3 as the trans ligand. We thus wished to see if [Os(NH3)5(SO2)] 2+ ions could be synthesized; if so, might they present with  1 -to- 2 linkage photo-isomerization in the solid state?…”

“…The most represented are the conflicting reports of [Os­(NH 3 ) 5 (H 2 O)] 2+ ions which may be unstable or long-lived . One reported attempt to coordinate SO 2 to osmium pentaammine produced an impure compound …”

The role of electrochemistry in the development of π-basic dearomatization agents

“…16 A solution of 2 was reduced in the presence of the pyridine derivative isonicotinamide (isn) in a variety of different solvents. The complex [Os(NH 3 ) 5 (isn)] 3+ was known to have a III/II reduction potential of −0.24 V (all potentials reported vs. NHE; 0.00 V NHE = −0.55 V vs. Fe(Cp) 2 ), 17 and therefore, the appearance of a reversible couple at this value in a cyclic voltammogram of 2 would signal the formation of [Os(NH 3 ) 5 (isn)] 2+ . Interestingly, for a variety of solvents believed at the time to be ineffective as ligands for Os(II) (e.g.…”