Metal nitric oxide complex
Metal nitrosyl complexes are complexes that contain nitric oxide, NO, bonded to a transition metal. Many kinds of nitrosyl complexes are known, which vary both in structure and coligand.
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.
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).
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