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