The complexes [M([8]aneSe₂][PF₆]₂ were prepared by the reaction of MCl₂ with two equivalents of [8]aneSe₂ (1,5-diselenacyclooctane) and TlPF₆ in MeCN, [M([8]aneSe₂)Cl₂] (M = Pt or Pd) by reaction of MCl₂ with [8]aneSe₂ in MeCN, and [M([8]aneSe₂)Br₂] (M = Pt or Pd) by the reaction of [MBr₂(MeCN)₂] and [8]aneSe₂ in MeNO₂. The single crystal X-ray structure of [Pd([8]aneSe₂)Cl₂]0.5MeNO₂ reveals Pd(II) in a square planar geometry with an Se₂Cl₂ donor set, [8]aneSe₂ adopting the chair-boat conformation. The complexes trans-[Pt([16]aneSe₄)X₂][PF₆]₂ (X = Cl or Br) and trans-[Pt([8]aneSe₂)₂Cl₂][PF₆]₂ were prepared from the Pt(II) analogues and X₂/CCl₄. The single crystal X-ray structure of trans- [Pt([16]aneSe₄)Cl₂][PF₆]₂ shows an Se₄Cl₂ donor set; [16]aneSe₄ adopts the up,up,down,down configuration. Treatment of MCl₂ (M = Pd or Pt) with [24]aneSe₆ (1,5,9,13,17,21-hexaselenacyclotetracosane) and TlPF₆ affords the complexes [M([24]aneSe₆)][PF₆]₂. Reaction of CoX₂ (X = Cl, Br or I) with [16]aneSe₄ (1,5,9,13-tetraselenacyclohexadecane) and NH₄PF₆ (or NaBPH₄ for X = Br) in MeNO₂ yield the [Co([16]aneSe₄)X₂][Y] complexes. The [M([16]aneSe₄)X₂]⁺ species (M = Rh or Ir; X = Cl or Br) were prepared by reaction of MX₃.3H₂O and [16]aneSe₄ in EtOH, or for [Rh([16]aneSe₄)Br₂]⁺, from [Rh(H₂O)₆]³⁺, LiBr and [16]aneSe₄, and isolated by the addition of either NaBF₄, NH₄PF₆ or NaBPh₄. The compound [Rh([8]aneSe₂)₂Cl₂][BF₄] was prepared from [{Rh(CO)₂Cl}₂], [8]aneSe₂ and HBF₄ in Me₂CO. The UV/vis and ⁵⁹Co NMR studies on [Co([16]aneSe₄)X₂]⁺ reveal that for X = Cl or Br only trans dihalogeno species are present whilst cis and trans forms are present when X = I. ⁷⁷Se-{¹H} NMR studies on [M([16]aneSe₄)X₂]⁺ (M = Rh or Ir; X = Cl or Br) and [Rh([8]aneSe₂)₂Cl₂}⁺ show the presence of both cis and trans forms in DMF solution, although for [M([16]aneSe₄)Cl₂]⁺ only the trans form is soluble in MeNO₂. The single crystal X-ray structures of [M([16]aneSe₄)Br₂][BPh₄] (M = Co or Ir) reveal Se₄Br₂ donor sets; [16]aneSe₄ adopts the up,up,down,down configuration.