| Abstract
| - The nature of the bonding in the binary transition-metal carbonyl complex has been analyzed by topologicalapproaches (atoms in molecules (AIM) and electron localization function (ELF)) from a series of calculationscarried out at the hybrid Hartree−Fock/DFT level (B3LYP). It is shown that the interaction between a transitionmetal and CO should be characterized as a dative bond, in which the monosynaptic basin of the carbon playsthe role of the disynaptic basin connecting the metal core to the carbon atom. For all atoms except Cr, Mn,and Cu, the multiplicity of the ground state is given by applying Hund's rule to the maximal core occupancy(i.e., [Ar]cn+2): high-spin complexes for n< 4, low-spin for n> 5, spin-conserved for n = 4, 5, 9. Thecharge transfers and the spin density on the ligand are rationalized by resonance structures of the samemultiplicity. In all complexes except CrCO and CuCO, the ELF function in the core has a local cylindricalsymmetry that in turn favors a linear structure; moreover, 2 electrons are available for the charge transfertoward the CO moiety and for the metal nonbonding valence basin. In CrCO and CuCO whose cores havea spherical symmetry, only one electron can be shared by the net transfer and the nonbonding valence basin.The maximization of the charge transfer implies a bent geometry. Finally, we propose two new donation−back-donation schemes based on the AIM and ELF partitions. In the ELF framework, the net charge transferis almost equal to the π back-donation, the σ-donation being negligible.
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