Is the CO frequency shift a reliable indicator of coumarin binding to metal ions through the carbonyl oxygen?

The coumarin ligand, 4-hydroxy-3-nitro-2H-chromen-2-one (Hhnc) and its Cu(II) and Ag(I) complexes were studied by DFT calculations at B3LYP/B1 and PW91/B1 levels. MEP of the deprotonated ligand, hnc−, and energy calculations of model metal complexes predicted the ligand binding to the metal ion through the hydroxyl and the nitro oxygens in agreement with experiment. Based on precisely selected Cu/Ag model complexes with hnc−, a relation between the vibrational behaviour of the ligand donor groups and the ligand binding modes in the complexes was deduced. The observed carbonyl ν(CO) downshift (50-90 cm−1) is attributed to intermolecular H-bonding formed between the CO group and lattice water molecules or due to the CO binding to the metal ion in case of bridging coumarin ligand (in Aghnc). Much larger ν(CO) downshift (∼220-240 cm−1) is predicted in case of monodentate or bidentate (with the nitro group) bonding of the carbonyl CO group to the metal ion.