text: This effect occurs with class-a metals such as iron(II).  The Cyclopentadienyl complex (CH)Fe(CO)(SCN) is an example of chemical symbiosis. The cyclopentadienyl directs the thiocyanate to bond through its softer Sulphur donor.  A more definitive example are the halopentamminocobalt(III) ions, Co(NH)X, which are more stable when the halogen, X, is fluoride than with iodide, and the halopentcyanocobalt(III) ions, Co(CN)X, which are most stable when the halogen is iodine.