Heterotrinuclear complexes containing a hexaselenolato-zinc(II), -cadmium(II), -nickel(II) or -iron(II) core: crystal structures of [(OC)3Fe(?-SePh)3M(?-SePh)3Fe(CO)3] (M = Zn, Cd, Ni or Fe) and [Fe(CO)2(phen)(SePh)2] (phen = 1,10-phenanthroline)