Crystal field theory with its assumptions of completely electrostatic metal-ligand interactions, does not appear to retionalize the spectrochemical series particularly well. To explain the order of ligands in the series we must admit some covalent contribution to the metal ligand bond, both sigma and pi. Sigma bonds are formed by ligands using their p orbitals (e.g. `Cl^(-)`) or by hybrid orbitals (e.g. `sp^(3)`) by `(H_(2)O`) since all ligands are capable of such sigma interaction, this is not very useful in rationalizing spectrochemical series. The pi bonding ability of ligands partially justifies their position in spectrochemical series. pi-acid ligands. which can accept electron density from filled metal orbital into their empty orbital via pi interation appear towards stronger/higher and of the series e.g. `CN^(-).PR_(3)` etc. While sigma-donor ligands are typically weak field ligands e.g. `Cl^(-),O^(2-)` etc. Q. Which of the following is false for CO ligand?A. it is a pi-acid ligand.B. CO accepts electron density from central atom into its pi bonding molecular orbital.C. CO forms sigma bond with central atom using its sp hybrid orbital.D. CO is a strong field ligand.