Bond distances are not always what they appear to be: discovery and un-discovery of the longest Cr(v)N triple bond

Abstract

The coordination chemistry of CrCl₃ with three pentadentate ligands having the [1,4,7]-triazacyclononane-1,4-diacetato motif have been investigated. The new resulting six-coordinate Cr(III)-chloro species react cleanly with sodium azide to form the corresponding azido species, which undergo photolysis under irradiation at 419 nm in acetonitrile–water solution to form Cr(V)-nitrido species that are partially hydrolyzed to their corresponding Cr(III)-hydroxo counterparts. Five of these Cr complexes have been characterized by X-ray crystallography. The hydroxo and nitrido species co-crystallize complicating crystal structural refinement. What at first appeared to be the longest CrN triple bond distance yet observed (1.66 Å) was found after re-refinement to be an artifact of positional disorder of the Cr atoms of 73% nitrido and 27% hydroxo species. The re-refined CrN bond distance is estimated as 1.58 Å, which agrees well with the observed CrN stretching frequency of 971 cm⁻¹ found by IR spectroscopy. UV-vis data for the “nitrido” complexes suggest that they are all three partially hydrolyzed. These results emphasize the care that must be taken in the characterization of compounds that may co-crystallize with structurally similar analogs.