Reaction of [M(CO)₄(nbd)] (M = Cr or Mo, nbd = norbornadiene) or [W(CO)₄(TMPA)] (TMPA = N,N,N′,N′-tetramethyl-1,3-propanediamine) with [8]aneSe₂ (1,5-diselenacyclooctane) yielded the cis-disubstituted tetracarbonyl species [M(CO)₄([8]aneSe₂)] (M=Cr, Mo or W). The complexes [M′X(CO)₅] (M′=Mn, X=Cl, Br or I; M′=Re, X= Cl or Br) reacted similarly with [8]aneSe₂ to give fac-[M′X(CO)₃([8]aneSe₂)] in high yield. Infrared and multinuclear NMR spectroscopic studies confirmed these assignments and indicated a single species in solution; δ(⁵⁵Mn) lies in the same range as observed for other fac-[MnX(CO)₃(diselenoether)] complexes, while for all of the compounds δ(⁷⁷Se) is to low frequency of [8]aneSe₂ itself (δ 137). Crystal structures of [W(CO)₄([8]aneSe₂)], [MnBr(CO)₃([8]aneSe₂)] and [ReBr(CO)₃([8]aneSe₂)] show the cyclic diselenoether chelating and adopting a chair-boat conformation. The compounds [16]aneSe₄ (1,5,9,13-tetraselenacyclohexadecane) and L (1,6-diselena-3,4-benzocyclononane) reacted with the metal(II) species [{MoBr₂(CO)₄)}₂] or [MI₂(CO)₃(NCMe)₂] in CH₂Cl₂ solution to give seven-co-ordinate [{MoX₂(CO)₃}₂([16]aneSe₄)], [WI₂(CO)₃([16]aneSe₄)], [MoX₂(CO)₃(L)] and [WI₂(CO)₃(L)], although these species decompose rapidly in co-ordinating solvents. Reaction of [16]aneSe₄ with two molar equivalents of [M′Cl(CO)₅] yielded the dinuclear complex [{MnCl(CO)₃}₂([16]aneSe₄)] (in which the tetraselenoether is thought to bind in a bidentate manner to each Mn) and the mononuclear complex [ReCl(CO)₃([16]aneSe₄)] (which is thought to involve bidentate ligation to [16]aneSe₄ with two free Se donors). The cationic species fac-[Mn(CO)₃(η³-[16]aneSe₄)]CF₃SO₃ was generated by treatment of [MnCl(CO)₅] with AgCF₃SO₃ in Me₂CO followed by addition of [16]aneSe₄.