The strongest CO binding and the highest C-O stretching frequency

A coupled-cluster study is performed on CO bound BeY complexes (Y = O, CO₃, SO₄, NH, NCN, and NBO) to understand the effect of attached ligands (Y) on the CO binding ability and C-O stretching frequency (ν_CO). Herein, we report that BeNCN has the highest CO binding ability (via both C- and O-side binding) among the studied neutral Be-based clusters, whereas OCBeSO₄ has the highest ν_CO among the neutral carbonyls. The nature and extent of shift in ν_CO compared to free CO are explained in terms of change in polarization in the bonding orbitals of CO and relative contribution from OC→BeY or CO→BeY σ-donation, and OC←BeY or CO←BeY π-back-donation. The largest blue-shift in OCBeSO₄ and the largest red-shift in COBeNH are consequences of the smallest OC←BeSO₄ π-back-donation and the largest CO←BeNH π-back-donation, respectively.