An attempt has been made to analyze the spin transition energies of Cr, a representative
example of transition metals, in a variety of complexes formed at regular (001) surfaces of
MgO, as well as the adsorption of CO by means of hybrid density functional theory
calculations and embedded cluster models. Clusters of moderate sizes are embedded in the
simulated Coulomb fields that closely approximate the Madelung fields of the host surfaces.
While the spin states of Cr are reduced and preserved in all complexes, be they defect-free
or-defect containing, the combined effects of the adsorbate and the substrate in the defect-free
OC.Cr.Mg9O13O2− complex were strong enough to favor the low-spin state and to quench the
spin. The deposited Cr atoms enhance the adsorption of CO. The significant weakening of
bond strength between OC and Cr in complexes supports the concept of bond order
conservation. The natural bond orbital (NBO) analysis reveals that the electronic structure of
the adsorbed metal represents a qualitative change with respect to that of the free metal. The
effects of spin contamination on the geometry, Mulliken charges and adsorption energy are
examined. The binding of CO precursor is dominated by the E (i )Cr.CO pairwise additive
components, and the role of the support was not restricted to supporting the metal. Relations
are established between the process of spin transition and the energy gaps between frontier
orbitals. The results show that the spin state of adsorbed metal atoms on oxide supports and
the role of precursor molecules in the properties of spin transition energies of Cr in complexes
need to be explicitly taken into account.