Ammines of osmium

Chapter XIII. Metal - Ammines of Osmium, Iridium, and... 

All three elements form complex ammino-derivatives. Those of osmium have been very little investigated those of iridium are analogous to the anunino-derivatives of platinum on the one hand and to the ammincs of cobalt and chromium on the other whilst the platinum derivatives resemble those of cobalt, save that the metal in the platinic derivatives is tetravalent and not trivalent as in the cobalt-ammines. 

Osmyl Tetra-ammine Chloride, 0sO2(NH3)4Cl2, and is obtained as a yellow, crystalline precipitate when ammonium chloride is added to aqueous potassium osinate.6 It is but slightly soluble in water, and boiling water decomposes it into osmium tetroxide and osmium di-ammine hydroxide. Its solution gives a violet coloration with potassium ferrocyanide, a reaction that may be utilised for the detection of osmium.7 It is insoluble in alcohol and in hydrochloric acid. When heated it decomposes completely, yielding a mass of spongy osmium. 

There are now a substantial number of nitrido complexes of osmium(VI) and osmium(IV) as well as the osmiamate ion [OsVII 03N]. In addition to the ammine and ethylenediamine complexes, much recent work has been carried out on the bipy, phen and terpy complexes, often in connection with research into the photodissociation of water. The nitrosyl chemistry of the element, though seemingly not as extensive as that of ruthenium, has received much attention, and there has been considerable work on the phosphine, arsine and stibine complexes. 

Nitrogen is, after oxygen, the most frequently encountered donor atom in the coordination chemistry of osmium. There is a large and growing body of work on the ammine, pyridine, ethylenediamine and porphyrin complexes, but the most popular and rapidly growing field of study at the present time is that of the polypyridyls , the 2,2 -bipyridyl, 1,10-phenanthroline and 2,2,2,6 - terpyridyl complexes of the metal. 

Ammonia forms a number of complexes with osmium (see Table 5), though the only fully established unsubstituted osmium ammine which has been isolated is the hexaammine [Os(NH3)6]3+ there is, however, electrochemical evidence for the existence both of [Os(NH3)6]2+ and of [Os(NH3)6]4+. Amongst the substituted ammines which have been isolated and characterized are several of osmium(II), all with supporting (or stabilizing) ir-acceptor ligands, e.g. [Os(N-H3)sCO]2 [Os(NH3)s(NO)]3+ and [Os(NH3)5(N2)]2+. 

Until recently ammine complexes of osmium have been little studied compared with their ruthenium analogs. This appears to have been caused by the lack of suitable synthetic routes to them. The discovery of pentaammine(dini-trogen)osmium (II) opened convenient routes to pentaammines of osmium (III), and a convenient synthesis of hexaammineosmium(IIl)2 gave new routes to the previously unknown nitrosyls of osmium(II). Here are given the synthesis of [Os(NH3)s(N2)]l2 and its conversion to [Os(NH3)5l]l2 the synthesis of [Os(NH3)6]l3 and its conversion to [Os(NH3)5(NO)]X3 H20(X = Cl, Br, I) and the preparation of [OsX(NH3)4(NO)]2+(X = OH, Cl, Br, I) from [Os(NH3)5(NO)]3+. 

Within the osmium complexes in oxidation states (II-IV) [11,12] the stability of the +4 oxidation state becomes more important. Ammine and tertiary phosphine complexes have been selected for detailed examination. 

Many of the ammine complexes are osmium(III) compounds the +2 state is less stable than with ruthenium, as expected, and osmium(II) compounds... 

Fig. 3. (A) Stereo pair of starfish sperm chromatin fibers stained with osmium ammine-B. Bracket indicates the fine deposition of stain on nucleosomes and linker DNA. Arrowheads indicate where fibers enter and exit the plane of the section, and asterisks indicate sharp bends in the fibre axis. (B) Stereo pair of a reconstructed volume of a starfish sperm head by EM tomography. Axes of some fibers have been marked. Arrowhead indicates where individual fibers cannot be distinguished (from Ref. [27]). Scale bar 100 nm.

---