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By means of the Interaction Region Indicator (IRI) and Quantum Theory of Atoms in Molecules (QTAIM), the influence exerted by NH2 (amino) and CN (cyano) as electron donor and electron acceptor substituent groups, respectively, located at para–positions of axial and equatorial pyridine rings of derivatized complexes coming from the [(PY5Me2)MoO]+ complex during the hydrogen molecular release in the gas phase was analyzed. In any case, a H–H covalent bond is forming at the transition state, with a strengthening of the electron density of 5.5% when the substituent group involved is NH2 at the para–position of the axial pyridine ring. However, there was no difference between NH2 and CN when these substituent groups are located at the para–positions of the equatorial pyridine rings. The topological properties of electron densities from the QTAIM are not perturbed by the electron donor and electron acceptor nature of the substituents, even when these substituent groups are located at the axial or equatorial pyridine rings of the Mo–based complex.
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•Combined QTAIM/IRI Topological Analysis for quantifying the capability to release molecular hydrogen.•Evaluating the effect of Axial/Equatorial positions of NH2 and CN substituents on the molecular hydrogen release.•A strong H–H polar covalent interaction is clearly revealed.•The molecular hydrogen release occurs without mattering the substituent group's nature and its axial or equatorial location.•The link between topological analysis and rate constants is not clear nor evident.