Alkyl- and aryl-selenotin compounds react with pentacarbonyl halides of manganese and rhenium to produce the dimeric [M(CO)4SeR]2 species, in marked contrast to the corresponding reactions of tin–sulphur compounds, which produce the tricarbonyls [M(CO)3SR]n. Similar tricarbonyls [M(CO)3SeR]n, believed to contain a cluster of metal atoms, are obtained by pyrolysis of the tetracarbonyl species.