Four heterobimetallic Fe–M (M = Cu^I, Ag^I or Hg^II) complexes and a mononuclear iron(II) complex containing a pair of strained planar four-membered chelate rings have been synthesized via the reaction of a neutral organometallic trans-Fe(CO)₃(dppy)₂ L [dppy = 2-(diphenylphosphino)pyridine] with the corresponding metal perchlorates. Their structures have been determined by single-crystal X-ray analysis. In [CuL(Me₂CO)]ClO₄·1.75Me₂CO 1, [CuL(H₂O)]ClO₄·CH₂Cl₂2, [AgL(dppy)]ClO₄·0.5MeOH 3, [HgL(H₂O)(OClO₃)]ClO₄· 2.5H₂O·0.5MeOH 4 and cis-[Fe(CO)₂(dppy-P,N)₂][ClO₄]₂·CH₂Cl₂5, the Fe–M (M = Cu, Cu, Ag or Hg) distances are 2.501(2), 2.512(2), 2.760(1) and 2.545(1)Å, respectively, unequivocally indicating the existence of a dative Fe→M bond. To our knowledge, 3 is the first binuclear iron(0)–silver(I) donor–acceptor complex to be fully characterized by single-crystal X-ray analysis, and 5 provides the first example of a first-row transition-metal complex which incorporates two planar four-membered chelate rings involving P and N donor atoms.