High-valent isocyanide complexes. Coexistence of strong π-donor and formal π-acceptor ligands in chromium(V) nitride complexes

The first examples of a metal complexes combining isocyanide ligands with the nitride ligand, namely trichloro(methylisocyanide)nitridochromate(V) (1) and dichlorobis(methylisocyanide)nitrido–chromium(V) (2) are reported. These complexes are prepared by ligand substitution on [Cr(N)Cl4]2− and the former has been structurally characterized as its tetraethylammonium salt. In 1, the CrN distance at 1.5510(9)/1.5497(10) Å is within the normal range for Cr(V)N bonds. The chromium–isocyanide bond length at 2.063(1) Å is shorter than CrCN bond lengths in cyanide complexes of nitrido chromium(V), but significantly longer than CrC bond lengths in homoleptic isocyanides of Cr(II) and Cr(0). The σ-donation of methylisocyanide coordinated to the nitride–Cr(V) moiety is comparable to that of amine ligands with ΔCH3NCΔCH3NC ≈ 20,800 cm−1. UV–vis spectroscopy further allows for the quantification of the excited state CrN stretching frequency to be ca. 820 cm−1 in the 2E(dxz, dyz, C4v) excited state of both 1 and 2. Reversible binding of the second methylisocyanide ligand is observed spectroscopically and found to be quite weak with K2 = 6 × 10−3 for the equilibrium [Cr(N)Cl3(CNCH3)]− + CNCH3 = [Cr(N)Cl3(CNCH3)2] + Cl−. Lability of the isocyanide ligands in 1 and 2 is demonstrated by EPR spectroscopy employing competition with trimethylphosphite.

► First examples of mixed nitride–isocyanide complexes.
► Lack of π-backbonding in high-valent isocyanide systems.
► Pseudo-linear electronic structure of Cr(V)–nitrides.