Dative and electron-sharing bonding in transition metal compounds

Quantum chemical calculations using density functional theory at the BP86-D3(BJ)/def2-TZVPP level of theory are reported for transition metal compounds [TM]-L in high and low oxidation states involving carbene, carbyne, alkene, and alkyne ligands L. The nature of the [TM]-L bond is analyzed with the energy decomposition analysis - natural orbitals for chemical valence (EDA-NOCV) method. The calculations reveal that transition metal compounds with ligands, that are typically classified as donor–acceptor complexes possessing dative bonds (Fischer-type carbenes and carbynes, alkene, and alkyne complexes) or as TM compounds with electron-sharing bonds (Schrock-type carbenes and carbynes, metallacyclopropanes, and metallacyclopropenes), exhibit significant differences between the orbital interactions when closed-shell or open shell fragments are used. Fischer-type carbene complexes have much lower orbital interaction (ΔE_orb) values when singlet fragments are employed compared to triplet fragments. In contrast, singlet and triplet fragments of Schrock-type carbene complexes give similar ΔE_orb values. The best description for Fischer-type carbyne complexes is found for neutral fragments in their electronic doublet state, which engage in a mixture of dative bonding (σ donation and π backdonation) and one electron-sharing π bond. The EDA-NOCV calculations of Schrock-type carbynes using open-shell species in their quartet electronic state give similar ΔE_orb values as neutral fragments in their electronic doublet state. Alkene and alkyne complexes, but also metallacyclic species, are best described with singlet fragments, but the difference between the ΔE_orb values for dative bonding and electron-sharing bonding using triplet fragments becomes much smaller for molecules that are considered as metallacycles. © 2018 Wiley Periodicals, Inc.