# Comparing the Electronic Structure and Hydride Atom Transfer Reactivities of Nickel(III) vs Cu(III) Complexes

## TL;DR

This study compares nickel and copper complexes in terms of their electronic structure and hydride transfer reactivity, finding nickel complexes to be significantly more reactive.

## Contribution

The paper introduces a new comparative analysis of Ni(III) and Cu(III) complexes, revealing distinct hydride transfer mechanisms and reactivity differences.

## Key findings

- Ni(III) complexes exhibit significantly faster hydride transfer reactions compared to Cu(III) complexes.
- X-ray absorption studies confirmed a one-unit oxidation state change in both Ni(II) to Ni(III) and Cu(II) to Cu(III) complexes.
- The higher reactivity of Ni(III) is attributed to a greater redox driving force compared to Cu(III).

## Abstract

NiIII (1-ox) and CuIII (2-ox) species, supported
by a bis-amidate-dioxime ligand scaffold,
were synthesized via one-electron oxidation of NiII (1) and CuII (2) using ceric ammonium
nitrate in methanol at −40 °C. These species were extensively
characterized by various spectroscopic tools, including X-ray absorption
spectroscopy. X-ray structural analysis revealed that NiII and CuII complexes adopt a similar geometry around the
metal center, while the CuIII complex exhibited significantly
shorter metal–ligand bond distances in the solid state relative
to CuII. X-ray absorption near-edge structure (XANES) studies
showed an energy shift of 0.65 eV at normalized 0.5 absorption between 1 (8343.42 eV) and 1-ox (8344.07 eV), whereas
oxidation of 2 (8979.40 eV) to 2-ox (8981.09
eV) resulted in a shift of 1.65 eV, confirming a one-unit oxidation
state change. The electrochemical analysis demonstrated that the NiIII/NiII redox couple is anodically shifted by ca.
350 mV compared to the CuIII/CuII potential.
The reactivity of 1-ox and 2-ox with BNAH,
an NADPH analog, were further analyzed, and kinetic analysis confirmed
a hydride transfer (HT) pathway. The reaction of 1-ox was found ca. 11 times faster than that of 2-ox. Both
reactions exhibited a high primary kinetic isotope effect (1-ox: 7.3; 2-ox: 11.2). Additionally, the kinetics of 1-ox and 2-ox were examined with TEMPOH, indicating
a concerted proton–electron transfer (CPET) mechanism. The
reaction rate of 1-ox was significantly higher than that
of 2-ox. The enhanced HT/CPET reactivity of 1-ox relative to 2-ox is attributed to its greater redox
driving force. This work highlights a distinct HT mechanism involving
NiIII/CuIII species, diverging from the conventional
paradigm observed in many metal-oxo systems, where a rate-limiting
hydrogen atom transfer is followed by a rapid electron transfer.