Chemistry:Transition metal nitrate complex

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Short description: Compound of nitrate ligands
Structure of the metal nitrate complex [Rh(NO3)5]2-.[1]

In the area of inorganic chemistry, a transition metal nitrate complex is a coordination compound containing one or more nitrate ligands. Such transition metal nitrates are common starting reagents for the preparation of other compounds.[2] Some are intermediates in the extraction of metals from their ores.

Ligand properties

Nitrate is isostructural with but less basic than carbonate. Both nitrate and carbonate exhibit comparable coordination geometries. The nitrogen center of nitrate does not form bonds to metals. Being the conjugate base of a strong acid (nitric acid, pKa = -1.4), nitrate has modest Lewis basicity. Two coordination modes are common: unidentate and bidentate. Unidentate nitrate is classified as X ligand in the Covalent bond classification method. With respect to HSAB theory, it is classified as hard.

When bonded as a bidentate ligand, it is denoted κ2-NO3. Bidentate nitrate is classified as X-L ligand in the Covalent bond classification method. With respect to HSAB theory, it is classified as hard.

In some cases where it is bidentate, nitrate is bound unsymmetrically in the sense that one M-O distance is clearly bonding and the other is more weakly interacting.[2] The MO-N distances for the coordinated oxygen are about 10 picometers longer than the N-Oterminal bonds. This observation suggests that the terminal N-O bonds have double bond character.

With three terminal O atoms, nitrate can in principle bind metals through many geometries. Even though the ligand is written as MNO3, it only binds metals through oxygen atoms. Thus, monodentate nitrate is illustrated by [Co(NH3)5NO3]2+, equivalently written as [Co(NH3)5ONO2]2+. Homoleptic metal nitrate complexes generally have O,O'-bidentate nitrate ligands.

Coordination complexes

Most commonly encountered are the hydrates of transition metal nitrates. The anhydrous derivatives, especially those that are molecular, are simpler to describe and fewer in number.

Homoleptic metal nitrates and related compounds
Formula name comment
Ti(NO3)4 titanium(IV) nitrate eight-coordinate, volatile
Co(NO3)3 cobalt(III) nitrate octahedral, volatile
Cu(NO3)2 copper(II) nitrate planar, volatile
AgNO3 silver nitrate coordination polymer[3]

Hydrates

sample of ferric nitrate nonahydrate, which features the [Fe(H2O)6]3+ ion

Typical metal nitrates are hydrated. Some of these salts crystallize with one or more nitrate ligands, but most dissolve in water to give aquo complexes, often of the stoichiometry [M(H2O)6]n+.

Some common hydrated metal nitrates
Formula Color Structure of metal center Comment
Cr(NO3)3(H2O)6[4] violet octahedral
Mn(NO3)2(H2O)4 pale pink octahedral two unidentate nitrates in solid state
Fe(NO3)3(H2O)9[5][6] pale violet octahedral
Co(NO3)2(H2O)2[7] red octahedral tetra- and hexahydrates also known
Ni(NO3)2(H2O)4[8][9] green octahedral several hydrates are known
Pd(NO3)2(H2O)2[10] pale brown square planar
Cu(NO3)2(H2O)x[11][12][13][14] blue distorted octahedral multiple hydrates known (x = 1.5, 2.5, 3); Jahn–Teller distorted
Zn(NO3)2(H2O)4 colorless octahedral
Hg2(NO3)2(H2O)2[15] colorless linear (Hg–Hg) contains the Hg22+ dimer

Synthesis

Metal nitrate complexes are often prepared by treating metal oxides or metal carbonates with nitric acid. The main complication with dissolving metals in nitric acid arises from redox reactions, which can afford either nitric oxide or nitrogen dioxide.

Anhydrous nitrates can be prepared by the oxidation of metals with dinitrogen tetroxide (often as a mixture with nitrogen dioxide, with which it interconverts). N2O4 undergoes molecular autoionization to give [NO+] [NO3−], with the former nitrosonium ion being a strong oxidant. The method is illustrated by the route to β-Cu(NO3)2:

Cu + 2 N2O4 → Cu(NO3)2 + 2 NO

Many metals, metal halides, and metal carbonyls undergo similar reactions, but the product formulas can be deceptive. For example from chromium one obtains Cr(NO3)3(N2O4)2, which was shown to be the salt (NO+)2[Cr(NO3)5]2-.[16] The redox reaction of nitrosonium and the metal can give rise to metal nitrosyl complexes. In some cases nitronium ions (NO2+) are observed.[17]

In some cases, nitrate complexes are produced from the reaction of nitrogen dioxide with a metal dioxygen complex:[18]

Pt(O
2)(PPh
3)
2 + NO
2 → Pt(NO
3)
2(PPh
3)
2          (PPh3 = triphenylphosphine)

Reactions

Given nitrate's low basicity, the tendency of metal nitrate complexes toward hydrolysis is expected. Thus copper(II) nitrate readily dissociates in aqueous solution to give the aqua complex:

Cu(NO3)2 + 6 H2O → [Cu(H2O)6](NO3)2

Pyrolysis of metal nitrates yields oxides.[19]

Ni(NO3)2 → NiO + 2 NO2 + 0.5 O2

This kind of reaction has been used to incorporate metal oxides into various catalytic supports.

Nitrate reductase enzymes convert nitrate to nitrite. The mechanism involves the intermediacy of Mo-ONO2 complexes.[20]

References

  1. ↑ Vasilchenko, Danila; Vorobieva, Sofia; Baidina, Iraida; Piryazev, Dmitry; Tsipis, Athanassios; Korenev, Sergey (2018). "Structure and Properties of a Rhodium(III) Pentanitrato Complex Embracing Uni- and Bidentate Nitrato Ligands". Polyhedron 147: 69–74. doi:10.1016/j.poly.2018.03.017. 
  2. ↑ 2.0 2.1 Addison, C. C.; Logan, N.; Wallwork, S. C.; Garner, C. D. (1971). "Structural Aspects of Co-ordinated Nitrate Groups". Quarterly Reviews, Chemical Society 25 (2): 289. doi:10.1039/qr9712500289. 
  3. ↑ Lindley, P. F.; Woodward, P. (1966). "An X-ray investigation of silver nitrate: A unique metal nitrate structure". Journal of the Chemical Society A: Inorganic, Physical, Theoretical: 123. doi:10.1039/J19660000123. 
  4. ↑ Lazar, D.; Ribár, B.; Divjaković, V.; Mészáros, Cs. (1991). "Structure of Hexaaquachromium(III) Nitrate Trihydrate". Acta Crystallographica Section C: Crystal Structure Communications 47 (5): 1060–1062. doi:10.1107/S0108270190012628. 
  5. ↑ Hair, Neil J.; Beattie, James K. (1977). "Structure of Hexaaquairon(III) Nitrate Trihydrate. Comparison of Iron(II) and Iron(III) Bond Lengths in High-Spin Octahedral Environments". Inorganic Chemistry 16 (2): 245–250. doi:10.1021/ic50168a006. 
  6. ↑ H. Schmidt, A. Asztalos, F. Bok and W. Voigt (2012): "New Iron(III) Nitrate Hydrates: Fe(NO3)3·xH2O with x = 4, 5 and 6". Acta Crystallographica Section C: - Inorganic Compounds, volume C68, pages i29-i33. doi:10.1107/S0108270112015855
  7. ↑ Prelesnik, P. V.; Gabela, F.; Ribar, B.; Krstanovic, I. (1973). "Hexaaquacobalt(II) Nitrate". Cryst. Struct. Commun. 2 (4): 581–583. 
  8. ↑ Gallezot, P.; Weigel, D.; Prettre, M. (1967). "Structure du Nitrate de Nickel Tétrahydraté". Acta Crystallographica 22 (5): 699–705. doi:10.1107/S0365110X67001392. 
  9. ↑ Morosin, B.; Haseda, T. (1979). "Crystal Structure of the β Form of Ni(NO3)2·4H2O". Acta Crystallographica Section B: Structural Crystallography and Crystal Chemistry 35 (12): 2856–2858. doi:10.1107/S0567740879010827. 
  10. ↑ Laligant, Y.; Ferey, G.; Le Bail, A. (1991). "Crystal Structure of Pd(NO3)2(H2O)2". Materials Research Bulletin 26 (4): 269–275. doi:10.1016/0025-5408(91)90021-D. 
  11. ↑ Dornberger-Schiff, K.; Leciejewicz, J. (1958). "Zur Struktur des Kupfernitrates Cu(NO3)2·1.5H2O". Acta Crystallogr 11 (11): 825–826. doi:10.1107/S0365110X58002322. 
  12. ↑ Morosin, B. (1970). "The Crystal Structure of Cu(NO3)2·2.5H2O". Acta Crystallogr B26 (9): 1203–1208. doi:10.1107/S0567740870003898. 
  13. ↑ J. Garaj, Sbornik Prac. Chem.-Technol. Fak. Svst., Cskosl. 1966, pp. 35–39.
  14. ↑ Zibaseresht, R.; Hartshorn, R. M. (2006). "Hexaaquacopper(II) Dinitrate: Absence of Jahn-Teller Distortion". Acta Crystallogr. E62: i19–i22. doi:10.1107/S1600536805041851. 
  15. ↑ D. Grdenić (1956). "The Crystal Structure of Mercurous Nitrate Dihydrate". Journal of the Chemical Society: 1312. doi:10.1039/jr9560001312. 
  16. ↑ Addison, C. Clifford (1980). "Dinitrogen Tetroxide, Nitric Acid, and Their Mixtures as Media for Inorganic Reactions". Chemical Reviews 80: 21–39. doi:10.1021/cr60323a002. 
  17. ↑ Wickleder, Mathias S.; Gerlach, Frauke; Gagelmann, Steffen; Bruns, Jörn; Fenske, Mandus; Al-Shamery, Katharina (27 February 2012). "Thermolabile Noble Metal Precursors: (NO)[Au(NO3)4], (NO)2[Pd(NO3)4], and (NO)2[Pt(NO3)6]". Angewandte Chemie International Edition 51 (9): 2199–2203. doi:10.1002/anie.201106107. 
  18. ↑ Cook, Christopher David.; Jauhal, G. S. (1967). "Oxidation of coordinated ligands. Sulfato and nitrato complexes of platinum". Journal of the American Chemical Society 89 (12): 3066–3067. doi:10.1021/ja00988a057. 
  19. ↑ Criado, J.M.; Ortega, A.; Real, C. (1987). "Mechanism of the thermal decomposition of anhydrous nickel nitrate". Reactivity of Solids 4 (1–2): 93–103. doi:10.1016/0168-7336(87)80089-X. 
  20. ↑ Hille, Russ; Hall, James; Basu, Partha (2014). "The Mononuclear Molybdenum Enzymes". Chemical Reviews 114 (7): 3963–4038. doi:10.1021/cr400443z. PMID 24467397. 



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