 Furthermore, second coordination sphere hydrogen-bonding interactions give rise to faster reactions because H-bonds are stronger to the reduced carbene as compared to the precursor.Such H-bonding interactions can also facilitate chirality transfer in enantioselective carbene-transfer reactions.In order for the σ bond to be stabilized (typically with a bond order slightly less than 1), a back-bonding action from the π molecular orbital to the anti-bonding π* molecular orbital is necessary and the porphyrin ring serves as an electron π-symmetry "buffer" to ensure this interaction is obtained.