In studies of the chemistry of new copper−nitrosyl complexes supported by tris(3-(trifluoromethyl)-5-methylpyrazol-1-yl)hydroborate (TpCF
3
,CH
3
) and tris(3-mesitylpyrazol-1-yl)hydroborate (TpMs,H), significant
effects of the scorpionate ligand substituents on the properties of the {CuNO}11 unit were found that have
implications for environmental influences on similar species in biological and catalytic systems. The
copper(I) complexes TpMs,HCu(THF) and TpCF
3
,CH
3
Cu(CH3CN) were structurally characterized by X-ray
crystallography, and their respective CO and NO adducts were studied by FTIR, EPR, NMR, and/or UV−vis
spectroscopies in solution. Both nitrosyl complexes disproportionate in the presence of excess NO to N2O
and TpR,R‘Cu(NO2); an X-ray structure of the latter product supported by TpCF
3
,CH
3
was determined. Unlike
previously studied paramagnetic [CuNO]11 compounds that exhibit EPR signals with g < 2.0 and large A
NO
values at temperatures below ∼40 K (Ruggiero, C. E.; Carrier, S. M.; Antholine, W. E.; Whittaker, J. W.;
Cramer, C. J.; Tolman, W. B. J. Am. Chem. Soc.
1993, 115, 11285−11298), TpMs,HCu(NO) is EPR silent at
4.2 K and exhibits an NMR spectrum (238 K, toluene-d
8) with sharp signals. Peak assignments for the NMR
spectrum were deduced from integrated intensities, temperature-dependent isotropic shifts, and the nuclear
relaxation rates. The unique NMR spectral behavior for the TpMs,H complex, which only differs from those
of analogues with simple phenyl substituents by virtue of the shape of the substrate binding pocket enforced
by the mesityl methyl groups, suggests that caution should be exercised in characterizing such adducts in
proteins and heterogeneous systems; subtle environmental effects may determine the applicability of EPR
versus NMR methods. The electron-withdrawing effects of the trifluoromethyl substituents in TpCF
3
,CH
3
Cu(NO) perturb ν(NO) and the Cu(I) → NO MLCT energy in the respective FTIR and UV−vis spectra and
induce a significant slowing of its disproportionation rate. These results, in conjunction with those obtained
from kinetic and spectroscopic studies on the TpMs,H system, support a mechanism for the disproportionation
involving generation of the CuNO adduct from NO and the Cu(I) precursor in a preequilibrium step, followed
by electrophilic attack of a second NO molecule on the adduct that is rate-controlling.
