| Sumario: | Two crystal structures of the mononuclear copper(l)-nitrosyl complexes [Cu(L3)(NO)] (1) and [Cu(L3′)(NO)](ClO) (2) with the related coligands L3 (hydrotris(3-tert-butyl-5-isopropyl-1 -pyrazolyl)borate) and L3′ (tris(3-tert-butyl-5-isopropyl-1-pyrazolyl)methane) are presented. These compounds are then investigated in detail using a variety of spectroscopic methods. Vibrational spectra show v(N—O) at 1698 cm and v(Cu—NO) split at 365/338 cm for 1, which translates to force constants of 12.53 (N—O) and 1.31 mdyn/Å (Cu-NO), respectively. The weak Cu—NO force constant is in agreement with the observed instability of the Cu-NO bond. Interestingly, complex 2 with the neutral coligand L3′ shows a stronger N-O bond, evident from v(N—O) at 1742 cm. This difference is attributed to a true second coordination sphere effect, where the covalency of the Cu(l)—NO bond is not altered. The EPR spectrum of 1 is in agreement with the Cu(l)—NO(radical) electronic structure of the complexes, as obtained from density functional theory (DFT) calculations. In addition, an interesting trend between g(g) and the Cu—N—O angle is established. Finally, high-quality MCD spectra of 1 are presented and assigned using TD-DFT calculations. Based on the in-depth spectroscopic characterization of end-on bound NO to copper(l) presented in this work, it is possible to determine the binding mode of the Cu-NO intermediate of Cu nitrite reductase studied by Scholes and co-workers (Usov, 0. M.; Sun, Y.; Grigoryants, V. M.; Shapleigh, J. P.; Scholes, C. P., J. Am. Chem. Soc. 2006, 128, 13102-13111) in solution as strongly bent (~135°) but likely not side-on.
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