Two, Three, or Not to Be? Elucidating Multiple Bonding in d⁶ Pseudotetrahedral Oxo and Imide Complexes

Abstract

Late transition metal oxo and imide complexes play an important role in the catalytic functionalization and activation of small molecules. An emerging theme in this area over the past few decades has been the use of lower-coordination numbers, and pseudo-tetrahedral geometries in particular, to stabilize what would otherwise be highly reactive species. However, the bonding structure in d 6 oxo and imide complexes in this geometry is ambiguous. These species are typically depicted with a triple bond, however recent experimental evidence suggests significant empirical differences between these complexes and other triply bonded complexes with lower d-counts. Here we use a suite of computational orbital localization methods and electron density analyses to probe the bonding structure of isoelectronic d 6 Co(III) oxo and imide complexes. These analyses suggest that a triple bond description is inaccurate due to a dramatically weakened σ interaction.While the exact bond order in these cases is necessarily dependent on the model used, several metrics suggest that the strength of the metal-O/N bond is most similar to other formally doubly bonded complexes. File list (4) download file view on ChemRxiv Manuscript.pdf (2.81 MiB) download file view on ChemRxiv TOC.tif (858.29 KiB) download file view on ChemRxiv SI.pdf (1.36 MiB) download file view on ChemRxiv xyz Files.zip (24.38 KiB)

Enzyme-Like Hydroxylation of Aliphatic C–H Bonds From an Isolable Co-Oxo Complex

“…We initially optimized the structure of the intermediate 4 ox (Figure , leftmost structure), which reveals the expected pseudotetrahedral geometry. The Co–O bond length is 1.590 Å, which is ∼0.06 Å shorter than that predicted for 4 (Table ), consistent with the removal of a d-electron from a d z 2 orbital with some σ* character . This short Co–O bond length indicates a strong covalent bond, which likely facilitates the electron delocalization indicated by the EPR spectrum, as discussed above.…”

“…This stretching frequency is consistent with a strong Co–O multiple bond. Additionally, a short Co–O bond length of 1.655(3) Å is observed in the SXRD structure (Figure ), also consistent with a strong multiple bond …”

“…DFT analysis further supports this similarity. The DFT predicted Co–O bond lengths and stretching frequencies are very similar (Table ), and the computed delocalization index (DI, a measure of the covalent bond order) is identical for both bonds (Table ). Together, the DFT analysis and IR spectroscopic data support an almost identical bonding interaction between Co and O in complex 4 and PhB­( t BuIm) 3 Co III O despite the slightly different Co–O stretching frequencies.…”

“… c Delocalization index (see the Experimental Section). d The p K a value for the deprotonation of the Co III -hydroxide complex to form the Co III -oxo complex. e p K a values extrapolated to DMSO given that p K a values correlate linearly between many solvents. f Co II -hydroxide/Co III -hydroxide redox potential. g For the O–H bond in the corresponding Co II -hydroxide complex. h Measured in MeCN. i Measured in THF. j p K a value determined using the methods described in references and (see the Experimental Section and the SI). …”

To Bond or Not to Bond: Metal–Metal Interaction in Heterobimetallic Rare-Earth Metal–Silver Complexes

“…To analyze intermetallic bonding in 3-La and 3-Y , localized molecular orbitals (LMOs) were calculated by using the Pipek–Mezey and Foster–Boys method. LMOs are commonly used to analyze bonding in metal complexes and metal clusters. − In all cases, no bonds between the silver atom and La and Y could be located. For 3-Y , an additional Intrinsic Bond Orbital (IBO) analysis was performed, which also showed no bond between the two metal atoms.…”