Why is the imine (C=N) bond so weak?

Nitrogen is similar to carbon in terms of electronegativity and size. MO theory suggests that the more electronegative N would result in a lowering of the of the π-bonding energy, E(πCN), below the E(πCC) of ethylene. As I understand it, the MO energy trend should be:


* Eπ(CO) > Eπ(CN) > Eπ(CC) > Eπ\*(CO) > Eπ\*(CN) > Eπ\*(CC)


Meaning the trend in bond strength should be:


* formaldehyde (H2C=O) > azomethine (H2C=NH) > ethylene (H2C=CH2)


However, bond dissociation energy implies that the C=N bond is the weakest compared to C=C and C=O. <https://labs.chem.ucsb.edu/zakarian/armen/11---bonddissociationenergy.pdf>


* formaldehyde (H2C=O) = 732 kJ/mol
* ethylene (H2C=CH2) = 682 kJ/mol
* azomethine (H2C=NH) = 644 kJ/mol


