Transition metal nitrite complex
In
Structure and bonding
Bonding modes
Three
An example of chelating nitrite is [Cu(bipy)2(O2N)]NO3 – "bipy" is the
Focusing on electron-counting in monometallic complexes, O-bonded, N-bonded are viewed as 1-electron pseudohalides ("X-ligand"). The bidentate O,O-bonded is an "L-X ligand", akin to bidentate carboxylate.
With respect to HSAB theory, the N bonding mode is more common for softer metal centers. The O and O,O-bidentate modes are hard ligands, being found on Lewis-acidic metal centers.
The kinetically-favored O-bonded isomer [(NH3)5Co−ONO]2+ converts to [(NH3)5Co−NO2]2+. In its reaction with ferric porphyrin complexes, nitrite gives the O-bonded isomer, Fe(porph)ONO. Addition of donor ligands to this complex induces the conversion to the octahedral low-spin isomer, which now is a soft Lewis acid. The nitrite isomerizes to the N-bonded isomer, Fe(porph)NO2(L).[5]
The isomerization of [(NH3)5Co−ONO]2+ to [(NH3)5Co−NO2]2+ proceeds in an intramolecular manner.[6]
Homoleptic complexes
Several homoleptic (complexes with only one kind of ligand) complexes have been characterized by
Synthesis of nitrito complexes
Traditionally metal nitrito complexes are prepared by
Metal nitrosyl complexes undergo base hydrolysis, yielding nitrite complexes. This pattern is manifested in the behavior of
- [Fe(CN)5NO]2− + 2 OH− → [Fe(CN)5NO2]4− + H2O
Reactions of nitrito complexes
Some anionic nitrito complexes undergo acid-induced deoxygenation to give the
- [LnMNO2]− + H+ → [LnMNO]+ + OH−
The reaction is reversible in some cases. Thus, one can generate nitrito complexes by base-hydrolysis of electrophilic metal nitrosyls.
Nitro complexes also catalyze the oxidation of alkenes.[12]
Bioinorganic chemistry
Metal nitrito complexes figure prominently in the nitrogen cycle, which describes the relationships and interconversions of ammonia up to nitrate. Because nitrogen is often a limiting nutrient, this cycle is important. Nitrite itself does not readily undergo redox reactions, but its metal complexes do.[13]
Oxidation to nitrate
The molybdenum-containing enzyme nitrite oxidoreductase catalyzes the oxidation of nitrite to nitrate:
- NO−2 + H2O → NO−3 + 2 H+
Reduction
The heme-based enzyme nitrite reductase catalyzes the conversion of nitrite to ammonia. The cycle begins with reduction of an iron-nitrite complex to a metal nitrosyl complex.[3]
The copper-containing enzyme
References
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- ISBN 9780470132395.
- ISBN 978-0-08-037941-8.
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- PMID 24694090.