A recent report of the structural and vibrational properties of heme-bound HNO in myoglolbin, MbHNO, revealed a long Fe-N-HNO bond with the hydrogen atom bonded to the coordinated N atom. The Fe-N(H)-O moiety was reported to exhibit an unusually high Fe-NHNO Stretching frequency relative to those of the corresponding {FeNO}(6) and {FeNO}(7) porphyrinates, despite the Fe-N-HNO bond being longer than either of its Fe-N-NO counterparts. Herein, we present results from density functional theory calculations of an active site model of MbHNO that support the previous assignment and clarify this seemingly contradictory result. The results are consistent with the experimental evidence for a ground-state Fe-N(H)-O structure having a long Fe-N-HNO bond and a uniquely high v(Fe-N(HNO)) frequency. This high frequency is the result of the correspondingly low reduced mass of the normal mode, which is largely attributable to significant motion of the N-bound hydrogen atom. Additionally, the calculations show the Fe-N(H)O bonding in this complex to be remarkably insensitive to whether the HNO and ImH ligand planes are parallel or perpendicular. This is attributed to insensitivities of the Fe-L-axial characters of molecular orbitals to the relative HNO and ImH orientation in both the parallel and perpendicular conformers.