A Combined Spectroscopic and Computational Study on the Electronic Structures of Cyclopentadienyl Bis-Phosphine...

Terminal transition-metal nitrides are key intermediates in dinitrogen activation and nitrogen-transfer chemistry and have therefore long attracted considerable attention. Despite the intensive work on Cr(V/VI) nitrides in the past, systematic investigations on Cr(IV) nitrides remain lacking. Herein, we report the synthesis and characterization of a Cr(IV) nitride, [Cp*Cr^IV(N)(depe)] (1, Cp* = C₅Me₅, depe = 1,2-bis(diethylphosphino)ethane), and its one-electron oxidized form, [Cp*Cr^V(N)(depe)]⁺ (2), using solid-state ¹⁵N nuclear magnetic resonance, electron paramagnetic resonance, and UV-vis spectroscopies coupled with density functional theory calculations. Our findings reveal that both species feature an overwhelming Cr≡N triple-bonding interaction that is yet considerably perturbed by the pronounced π-donating and π-accepting properties of the cyclopentadienyl and bisphosphine ligands, respectively. As a manifestation, Cr(IV) nitride 1 is distinguished by a rhombic ¹⁵N chemical shift tensor that is disparate from an axial one anticipated for a Cr≡N triple bond, because the Cr-to-depe π-backdonation lifts the 2-fold degeneracy of the Cr≡N π-bonding. Due to the presence of a low-lying LMCT excitation from the doubly occupied Cp* π-orbital to the singly populated Cr d_x2-y2 orbital, Cr(V) nitride 2 exhibits an unprecedented g_∥ > g_e > g_⊥ pattern for all d¹ metal oxides and nitrides whose three g-components are otherwise constantly less than g_e.