. (Middle) Sigma bond. (Bottom) Back-bond. Most complexes containing the NO ligand can be viewed as derivatives of the nitrosyl cation, NO+. The nitrosyl cation is
isoelectronic with
carbon monoxide, thus the bonding between a nitrosyl ligand and a metal follows the same principles as the bonding in
carbonyl complexes. The nitrosyl cation serves as a two-electron donor to the metal and accepts electrons from the metal via
back-bonding. The compounds
Co(NO)(CO)3 and
Ni(CO)4 illustrate the analogy between NO+ and CO. In an electron-counting sense, two linear NO ligands are equivalent to three CO groups. This trend is illustrated by the isoelectronic pair Fe(CO)2(NO)2 and [Ni(CO)4]. These complexes are
isoelectronic and, incidentally, both obey the
18-electron rule. The formal description of nitric oxide as NO+ does not match certain measureable and calculated properties. In an alternative description, nitric oxide serves as a 3-electron donor, and the metal-nitrogen interaction is a
triple bond.
Linear vs bent nitrosyl ligands 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−1, whereas bent nitrosyls absorb in the range 1525–1690 cm−1. 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)2(NO)Cl]+. The NO− is also common for alkali-metal or alkaline-earth metal-NO molecules. For example. LiNO and BeNO bear Li+NO− and Be+NO− 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 electron count in the metal-N-O
π system. Complexes more than 6 electrons in the system tend to have bent geometries at N. Thus, [Co(
en)2(NO)Cl]+, with eight electrons of pi-symmetry (six in t2g orbitals and two on NO, {CoNO}8), adopts a bent NO ligand, whereas [Fe(CN)5(NO)]2−, with six electrons of pi-symmetry, {FeNO}6), adopts a linear nitrosyl. In a further illustration, consider the {MNO} d-electron count of the [Cr(CN)5NO]3− anion. In this example, the cyanide ligands are "innocent", i.e., they have a charge of −1 each, −5 total. To balance the fragment's overall charge, the charge on {CrNO} is thus +2 (−3 = −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 fragment's overall charge of +2, to give 5. Written in the Enemark-Feltham notation, the d electron count is {CrNO}5, and the nitrosyl is linear. The results are the same if the nitrosyl ligand were considered NO+ or NO−. ==Representative classes of compounds==