Heteroatomic multiple bonded corrosion inhibitors: Coordination chemistry, bonding and synergistic behavior of σ-donors and π-acceptors

The synergistic effect of σ-donation and π-acceptance is crucial for corrosion inhibitors' effective coordination, adsorption and inhibition potential. The presence of heteroatomic multiple bonds, including >C=N–, >C=O, >C=S, –C≡N, –N=O and >S=O, performs dual functionalities as σ-donors and π-acceptors. They transfer their (π- and non-bonding) electrons in the metallic d-orbitals by a process known as donation, forming stable coordination complexes. The electron-deficient sites created by conjugation allow inhibitors to accept electrons through (back-bonding, backdonation, or retrodonation) into their vacant p-orbitals (π* orbitals). Heteroatoms in multiple bonds coordinate and bind to metal surfaces more efficiently than those in single bonds, making them better ligands and corrosion inhibitors. The present report explores the bonding mechanism, coordination chemistry, charge distribution, chelation and synergistic effect of σ-donation and π-acceptance of heteroatomic multiple bonds. The donation and retrodonation bonding of heteroatom multiple bonds have been schematically explained for the first time. Besides participating in the σ-donation, π-acceptance, charge distribution and chelation, the heteroatomic multiple bonded functional groups also participate in physisorption by becoming the charged species depending upon the nature of electrolytes. There has also been discussion of the difficulties and opportunities in various series of heteroatomic multiple-bonded compounds as corrosion inhibitors.