Why or why not can hydroquinone and 2H-pyrrole act as ligands?
Zaworotko, M. J.; Stamps, R. J.; Ledet, M. T.; Zhang, H.; Atwood, J. L. Heterocyclophane complexes of transition metals. 1. Synthesis and crystal structure of both the (.eta.5-2.2pyrroloparacyclophane)tricarbonylchromium and the (.eta.6-2.2pyrroloparacyclophane)tricarbonylchromium. Organometallics 1985, 4 (10), 1697-1700. DOI: 10.1021/om00129a001


A chromium tricarbonyl complex of a pyrrole is reported, here the pyrrole coordinates with all four carbon atoms and the nitrogen atom. I think that the pyrrole is a six electron donor to the chromium.


Bentivegna, B.; Mariani, C. I.; Smith, J. R.; Ma, S.; Rheingold, A. L.; Brunker, T. J. Formation, Stability, and Structures of Borenium and Boronium Cations Derived from Pentamethylazaferrocene–Boranes by Hydride or Chloride Abstraction Reactions. Organometallics 2014, 33 (11), 2820-2830. DOI: 10.1021/om500348u


A pyrrole binds with all five non hydrogen atoms to an iron which is also bonding to a pentamethyl cyclopentadienyl anion. This is forming a protonated version of CpFe(C4N) which looks very much like ferrocene. In this case the bonding is the same as it was in the chromium compound.


Yamamoto, K.; Kimura, S.; Murahashi, T. σ-π Continuum in Indole-Palladium(II) Complexes. Angew. Chem. 2016, 128 (17), 5408-5412. DOI: 10.1002/ange.201601992


A pyrrole binds with two carbons as a pi ligand to palladium. These selected results suggest to me that pyrrole can bind as an alkene or as if it was a Cp ligand to transition metals.


Son, S. U.; Reingold, J. A.; Carpenter, G. B.; Czech, P. T.; Sweigart, D. A. Charge-Assisted Hydrogen Bonding and Other Noncovalent Interactions in the Self-Assembly of the Organometallic Building Block [(η6-hydroquinone)Rh(P(OPh)3)2]+with a Range of Counteranions. Organometallics 2006, 25 (22), 5276-5285. DOI: 10.1021/om0604425


It reports the binding of a rhodium to all six carbon atoms of hydroquinone. Here it acts as a six electron donor.



So there is nothing to stop the molecules in the question acting as pi ligands to transition metals.