text: The M-N-O unit in nitrosyl complexes is usually linear, or no more than 15° from linear. In some complexes, however, especially when back-bonding is less important, the M-N-O angle can strongly deviate from 180°.  Linear and bent NO ligands can be distinguished using infrared spectroscopy. Linear M-N-O groups absorb in the range 1650–1900 cm, whereas bent nitrosyls absorb in the range 1525–1690 cm.  The differing vibrational frequencies reflect the differing N-O bond orders for linear (triple bond) and bent NO (double bond).
  The bent NO ligand is sometimes described as the anion, NO. Prototypes for such compounds are the organic nitroso compounds, such as nitrosobenzene. A complex with a bent NO ligand is trans-[Co(en)(NO)Cl]. The NO is also common for alkali-metal or alkaline-earth metal-NO molecules. For example. LiNO and BeNO bear LiNO and BeNO ionic form.
  The adoption of linear vs bent bonding can be analyzed with the Enemark-Feltham notation.  In their framework, the factor that determines the bent vs linear NO ligands is the sum of electrons of pi-symmetry.  Complexes with "pi-electrons" in excess of 6 tend to have bent NO ligands.  Thus, [Co(en)(NO)Cl], with eight electrons of pi-symmetry (six in t orbitals and two on NO, {CoNO}), adopts a bent NO ligand, whereas [Fe(CN)(NO)], with six electrons of pi-symmetry, {FeNO}), adopts a linear nitrosyl.  In a further illustration, the {MNO} d-electron count of the [Cr(CN)NO] anion is shown. In this example, the cyanide ligands are "innocent", i.e., they have a charge of &minus;1 each, &minus;5 total. To balance the fragments overall charge, the charge on {CrNO} is thus +2 (&minus;3 = &minus;5 + 2). Using the neutral electron counting scheme, Cr has 6 d electrons and NO· has one electron for a total of 7. Two electrons are subtracted to take into account that fragments overall charge of +2, to give 5. Written in the Enemark-Feltham notation, the d electron count is {CrNO}. The results are the same if the nitrosyl ligand were considered NO or NO.