Three-Coordinate Nickel and Metal-Metal Interactions in a Heterometallic Iron-Sulfur Cluster

Biological multielectron reactions often utilize metalloenzymes with heterometallic sites, such as anaerobic carbon monoxide dehydrogenase (CODH) which has a nickel-iron-sulfide cubane with an unprecedented three-coordinate nickel site. Here, we isolate synthetic iron-sulfur clusters with three-coordinate nickel, which also have tungsten in one vertex. EPR, Mössbauer, and SQUID data are combined with DFT computations to show how the electronic structure arises from magnetic coupling between the Ni, Fe, and W sites. X-ray absorption spectroscopy favors a description as nickel(I) in two oxidation levels. Spectroscopically validated density-functional theory (DFT) calculations indicate that two electrons are stored in a nonpolar Ni-W bond. Because of the Ni-W bond, the nickel(I) site does not have substantial unpaired spin density. This gives insight into previous measurements on CODH, and generally suggests that metalloenzymes could store redox equivalents and stabilize low-valent metal centers through metal-metal bonding.