More than 1000 mononuclear organoosmium compounds are described comprehensively in this volume. Two thirds of the volume are devoted to the carbonyl compounds except for those with one CO group which were already treated in volume A1. Most of about 700 compounds bear two to four CO groups and, in addition, heteroatom-bonded ligands in organic ligands such as alkyl, aryl, or acyl. H2Os(CO)4 and its salts play an important role in the synthesis of organoosmium compounds including homo- or heterometallic clusters. Os(CO)5 was only obtained in high-pressure reactions and is less stable than Fe(CO)5; therefore, it has attracted much less attention than its iron analogue. The last third of the volume contains the interesting chemistry of about 50 thio-, seleno-, and tellurocarbonyl compounds, more than 150 compounds with one to six isocyanide ligands, and more than 100 carbene and carbyne compounds. In most of the compounds dealt with in this volume, osmium is octahedrally sixcoordinate, but steric or electronic effects often cause trigonal-bipyramidal or even squareplanar fivefold coordination. Nearly all compounds are heteroleptic containing up to five different ligands. For that reason a great number of isomers are known. Substitution reactions generally proceed stereospecifically, thus allowing the synthesis of a great variety of new compounds. The most important reactions are given in reaction schemes of tables.