Three W VI crystal structures with multifarious metal−ligand bond types are used to theoretically predict homolytic metal−element bond enthalpies with 11 popular DFT functionals, MP2 wave function methods, and four common valence basis set/pseudopotentials in order to evaluate the accuracy and precision of the resultant bond enthalpy data. To our knowledge, for the first time, estimates of component metal−ligand σand π-bond strengths are computed. The WE (E = C, N, O) bond enthalpies have the consistent trend σ > second π > first π. In contrast, the element−element BDE trend for the 2p homologues is second π > first π > σ for nitrogen and oxygen, and σ > first π > second π for carbon. These differences may underpin the differences in stability trends and thus reactivity behavior for metal−element multiple bonds as compared to the element−element multiple bonds, and metal−element triple bonds versus their corresponding double bonded counterparts. For example, Odom et al.show that MeI nucleophilically attacks at the imide (MN) rather than the nitride (M ≡ N) ligand; the relative π-bond strengths derived herein provide a thermodynamic rationalization for this site preference. In this study, it is deduced from the calculated thermodynamics that the W−oxo ligand is more congruous with a triple bond than a double bond, consistent with the bonding model set forth in the seminal 1961 Ballhausen− Gray paper.

The contribution of metal identity to the activation and functionalization of methane by a series of three‐coordinate imide complexes is evaluated in silico for a 3‐by‐3 block of metals from Fe to Pt. Three mechanisms were studied: oxidative addition (OA) to the metal; hydrogen atom abstraction (HAA) by the imide nitrogen; and, [2+2] addition across the metal‐imide bond. In no studied case, was a [2+2] mechanism preferred, perhaps suggesting this mechanism is largely (entirely?) the domain of d0 imides. There is a diagonal relationship within the nonet of metals studied in that OA is preferred for earlier, heavier (5d) members of the series, transitioning to an HAA mechanism for later, lighter (3d) imides. DFT indicates that important parameters in partitioning between HAA and OA mechanisms include the strength of the metal‐imide π‐bond, the ability of larger metals to accommodate increases in formal oxidation state and coordination number, and the soft acid/base compatibility of larger transition metals with soft hydride and methyl ligands

This research evaluates 5d metal imide complexes, (OH)M═NMe (M = W, Re, or Os), and their reactions with methane to form dimethylamine. Each is calculated to follow a consistent reaction pathway regardless of the metal's d-orbital occupation, whereby the methane C-H bond undergoes oxidative addition (OA) to the metal and then the methyl migrates from the metal to the nitrogen to form an amide. Finally, hydrogen migrates to the nitrogen before dissociating to form amine products. While homolytic M-imide, M-amide M-H, and M-CH bond dissociation free energies (BDFEs) were analyzed, the BDFEs of neither hexavalent nor tetravalent metal moieties reflect the oxidative addition kinetics. Instead, a strain theory approach, supported by electron density analysis for the OA transition state, is found to be explanatory. Notably, the rate-determining step, the hydrogen migration transition state, has a consistent jump in free energy versus the preceding intermediate, (OH)M(H)-N(CH)Me, for all metals evaluated. Thus, the height of the RDS is largely reflective of the stability of the M-amide intermediate, suggesting a strategy for viable catalysis.

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