Bonding in metal carbonyls involves two complementary components acting together, described as a synergic effect. First, the carbon lone pair of carbon monoxide donates into an empty metal orbital, forming a sigma bond. Second, filled metal d orbitals donate electron density back into the empty antibonding pi* orbitals of CO, which is the pi back-bonding component. This back-donation strengthens the metal-carbon bond while simultaneously weakening the carbon-oxygen bond, an effect confirmed by lowered C-O stretching frequencies in infrared spectra. The two donations reinforce each other, hence the term synergic. Simple ionic attraction does not describe this covalent, orbital-overlap interaction. Hydrogen bonding is impossible because carbonyl carbon and oxygen offer no suitable hydrogen donors. Reduction of CO to carbide is not what occurs; the molecule stays intact while its bond order falls. This synergic model is the standard NCERT explanation for carbonyl stability. Plausibility check: the experimentally observed decrease in C-O bond order and stretching frequency on coordination directly supports back-donation into pi* orbitals, validating the synergic picture.