Figure 3.

Left: experimental wavenumbers $\tilde{\nu }$ of the N–O stretch of solid samples of 1–4 as a function of the N–O distance d. The reference line is a fit according to Badger's rule applied on the free NO^+/0/− species. Included are two known iron species: a [Fe(CN)₅(NO)]²⁻, b [Fe(H₂O)₅(NO)]²⁺. The “+” marks the calculated $\tilde{\nu }$ of 1′. Right: force constants f of the N−O bonds as a function of the NO's QTAIM charge q for 1–4, 1′, a, b.