Osmium trihalides

Pyridazines form complexes with iodine, iodine monochloride, bromine, nickel(II) ethyl xanthate, iron carbonyls, iron carbonyl and triphenylphosphine, boron trihalides, silver salts, mercury(I) salts, iridium and ruthenium salts, chromium carbonyl and transition metals, and pentammine complexes of osmium(II) and osmium(III) (79ACS(A)125). Pyridazine N- oxide and its methyl and phenyl substituted derivatives form copper complexes (78TL1979). 

It is a singular circumstance that the known chemistry of the tertiary phosphite complexes of osmium differs quite significantly from that of the tertiary phosphines, arsines and stibines. The closest analogue to P(OR)3 in osmium coordination chemistry would seem to be PF3, but even here the similarities are not marked. The oxidation states found are 0, II, III and IV (there are no established zerovalent unsubstituted osmium phosphine complexes), and the phosphites form unsubstituted species of the type OsL and [OsL ] " which have no counterparts in phosphine chemistry. The reason for these differences must be associated in part at least with the different cone angles and basicities of P(OR)3 ligands as against PR3. Further similarities and differences between the chemistries of osmium phosphines, phosphites and phosphorus trihalide complexes would obviously constitute a worthwhile study. 

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