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With reference to coordination compounds, most chelate rings formed with the metal atom and ligands are 5-member rings (including the central metal atom), such as complexes with Oxalate, ethylenediamine, dimethylglyoxime and glycinate ions as complexing agents.

Are there any compounds which contain stable 4-member chelate rings? Or any 7-member rings as well for that matter? I understand that angle strain makes these complexes hard to form but are there any exceptions to this trend?

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    $\begingroup$ The first thing that would come into my mind would be porphyrine based complexes. But I don't know if you were looking for something like that or if I've got your question wrong here. When I ask my students about chelate stability they often tell me about thermodynamic and kinetic stability but there is a third effect, macrocyclic stability. If the ring is already closed and stable but can coordinate different ring sizes are possible as well and square plane forms even without d8 configurations if necessary. $\endgroup$ – Justanotherchemist Feb 12 at 8:21
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    $\begingroup$ 4-rings are more or less common with huge central atoms (like uranium). $\endgroup$ – Ivan Neretin Feb 12 at 8:35
  • $\begingroup$ @IvanNeretin Could you provide some examples/references please? $\endgroup$ – Aniruddha Deb Feb 12 at 8:45
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Four-member rings can be formed with simple oxy-anion ligands such as nitrate. An example with iron as the central ion is given here.

Reference

  1. G. R. A Wyllie, O. Q. Munro, C. E. Schulz, W. R. Scheidt, "Structural and Physical Characterization of (Nitrato)iron(III) Porphyrinates [Fe(por)(NO3)] — Variable Coordination of Nitrate", Polyhedron, Oct 10 2007, 26(16): 4664-4672.
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    $\begingroup$ Yeah, acetate may also work like that if cation is small enough, like Zn. $\endgroup$ – Mithoron Feb 13 at 16:23

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