Platinum Metals Rev., 1967, 11, (3), 104
Carbonyl Complexes of Osmium
Triosmium Dodecacarbonyl And Its Derivatives
The study of the carbonyls of most of the platinum metals has led to the preparation of a whole series of new complexes, some of which have potential usefulness in the field of homogeneous catalysis. In the case of osmium carbonyls, however, research has so far been hampered by difficulties in their preparation. The new method of preparation described here now makes it possible to investigate the chemistry of osmium complexes along the same lines.
In recent years there has been a tremendous advance in the chemistry of the transition metals including those of the platinum group metals. The preparation and study of carbonyl complexes has proved most fruitful, and it is hoped that further investigation of reactions involving the carbonyls may result in the discovery of compounds having useful properties, particularly in the field of homogeneous catalysis. An improved method has now been developed for the preparation of triosmium dodecacarbonyl, Os3(CO)12, and some of its reactions have been studied.
Preparation of Osmium Carbonyls
In 1943 Hieber and Stallman (1) produced carbonyls of osmium by the action of carbon monoxide at high pressure either on osmium tetroxide or on osmium trihalides in the presence of silver or copper powder to take up the halogen. They separated two products, a volatile colourless liquid (m.p. − 15°C) and a yellow crystalline solid claimed respectively to be Os(CO)5 and Os2(CO)9. An X-ray examination (2) of the latter has, in fact, shown it to be Os3(CO)12, the molecule having three osmium atoms at the corners of an equilateral triangle, each osmium atom having four terminal CO groups, two approximately perpendicular to and two parallel to the plane of the triangle. Attempts to produce the carbonyls by either of these routes have resulted in very variable and very low yields, and, in the case where osmium tetroxide was the starting material, the formation of large quantities of black decomposition products. A new procedure (3, 4) has now been developed in these laboratories which results in the production of Os3(CO)12 in good yields (70 per cent) and, as a by-product, the oxycarbonyl, Os4O4(CO)12 (5). A solution of osmium tetroxide in xylene is heated at 175°C for 16 hr in an autoclave containing carbon monoxide at a pressure of 190 atmospheres. The large yellow crystals of Os3(CO)12 are readily separated by hand from the white oxycarbonyl Os4O4(CO)12 and both compounds are finally purified by recrystallisation from benzene. In the presence of xylene there are no signs of the decomposition products which appear when the reaction is carried out in its absence.
Reactions of Os3(CO)12
When Os3(CO)12 is reacted in hot xylene solution with triphenylphosphine [P(Ph)3] and with o-phenylenebisdimethylarsine (Diars) the substitution products [Os(CO)3 PPh3]3 and [Os(CO)2Diars]3 are formed. In both cases molecular weight determinations by osmometry have confirmed that the tri-nuclear metal cluster in the original carbonyl has been retained in the derivatives. This is similar to the behaviour of the analogous ruthenium carbonyl, Ru3(CO)12, with triphenylphosphine (6). Cleavage of the metal-metal bonds is, however, achieved if [Os(CO)3PPh3]3 is reacted in benzene solution with halogens, and compounds of the type Os(CO)3PPh3X2, where X = Cl, Br, or I, have been isolated.
Both the parent carbonyl and the triphenyl-phosphine derivative react with mercuric chloride to yield interesting new compounds containing Os-Hg bonds. The former reacts in hot xylene solution to yield (CO)4Cl Os-HgCl and the latter in cold benzene solution to yield (CO)3(PPh3)ClOs-HgCl.
Similar Os(O) and Os(II) complexes, e.g. Os(CO)3(PPh3)2 and Os(CO)2(PPh3)2Cl2 have been reported by Collman and Roper (7), who have also prepared cationic complexes containing Os-Hg bonds, e.g. [(CO)3(PPh3)2 Os-HgCl]HgCl3 (8). However, these compounds were produced by an entirely different route and the derivatives listed, together with some of their properties, are the first to have been prepared directly from Os3(CO)12.
Further work on the reactions of the osmium carbonyl and its derivatives with halogens, halogen acids and mercuric halides is proceeding and will be reported in due course.
The author’s thanks are due to Professor R. S. Nyholm for guidance and encouragement in this work.
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