Transition metal nitrile complexes

Transition metal nitrile complexes are coordination compounds containing nitrile ligands. Because nitriles are weakly basic, the nitrile ligands in these complexes are often labile. Nitriles are less complicated solvents than water because they are aprotic.[1]

[Cu(MeCN)4]+, often encountered as its PF6− salt, is a common transition metal nitrile complex.

Scope of nitriles

PdCl2(NCPh)2. Acetonitrile is the most popular ligand in this class. It is inexpensive and relatively basic for a nitrile. The coordination of many specialized nitriles have been investigated, e.g. Nitriles are weak pi-acceptor ligands. The structures of [Ru(NH3)5(NCPh)]n+ have been determined for the 2+ and 3+ oxidation states. Upon oxidation the Ru-NH3 distances contract and the Ru-NCPh distances elongate, consistent with ammines serving as pure-sigma donor ligands and nitriles functioning as pi-acceptors.[2]

Structural comparisons of [Ru(NH3)5(NCPh)]n+ for 2+ and 3+ salts. Distance in picometers.

Synthesis and reactions

Typical nitrile ligands are acetonitrile, propionitrile, and benzonitrile. These ligands are also popular solvents, which is usually the medium for the synthesis of their complexes. Because nitrile solvents have high dielectric constants, cationic nitrile complexes are often soluble in the nitrile.

Some complexes can be prepared by dissolving the anhydrous metal salt in the nitrile. In other cases, a suspension of the metal is oxidized with a solution of NOBF4 in the nitrile:[3]

Ni + 6 MeCN + 2 NOBF4 → [Ni(MeCN)6](BF4)2 + 2 NO

When applied to the hexacarbonyls of molybdenum and tungsten, the NO binds to the metal:[4]

M(CO)6 + 4 MeCN + 2 NOBF4 → [M(NO)2(MeCN)4](BF4)2
Portion of the structure of the tetrachlorozincate (ZnCl42−) salt of [Ni(MeCN)6]2+.[5]

Reactions

Transition metal nitrile complexes are usually employed because the nitrile dissociates rapidly and is chemically inert. Cationic nitrile complexes are however susceptible to nucleophilic attack at carbon. Consequently some nitrile complexes catalyze the hydrolysis of nitriles to give the amides.

Fe- and Co-nitrile complexes are intermediates in nitrile hydratase enzymes. N-coordination activates the sp-hybridized carbon center toward attack by nucleophiles, including water. Thus coordination sets into motion the catalytic hydration:

M-NCR + H2O → M-O=C(NH2)R
M-O=C(NH2)R + NCR → O=C(NH2)R + M-NCR

Examples

Many homoleptic nitrile complexes are isolated as salts with weakly coordinating anions.

  • Tetrakis(acetonitrile)copper(I) hexafluorophosphate ([Cu(MeCN)4]PF6), a colorless solid that serves as a source of naked "Cu+"
  • Bis(benzonitrile)palladium dichloride (PdCl2(PhCN)2), an orange solid that serves as a source of "PdCl2"
  • Hexakis(acetonitrile)nickel(II) tetrafluoroborate ([Ni(MeCN)6](BF4)2), a blue solid that is a source of naked "Ni2+"
  • Dimolybdenum deca- and octa-kis(acetonitrile) tetrafluoroborate ([Mo2(MeCN)8/10](BF4)4), a source of naked "Mo24+". Related Tc24+, Re24+, and Rh24+ complexes are also known.
  • Tricarbonyltris(propionitrile)molybdenum(0) (Mo(CO)3(C2H5CN)3), a source of "Mo(CO)3". Related Cr and W complexes are known.[6]

Complexes of η2-nitrile ligands

In some of its complexes, nitriles function as η2-ligands. This bonding mode is more common for complexes of low-valence metals, such as Ni(0). Complexes of η2-nitriles are expected to form as transient intermediates in certain metal-catalyzed reactions of nitriles, such as the Hoesch reaction and the hydrogenation of nitriles. In some cases, η2-nitrile ligands are intermediates that preceded oxidative addition.[7]

Structure of Ni(diphosphine)(η2-PhCN).[8]

See also
  • Cyanometalate - coordination compounds containing cyanide ligands (coordinating via C)

References
  1. Rach, S. F.; Kühn, F. E. (2009). "Nitrile Ligated Transition Metal Complexes with Weakly Coordinating Counteranions and Their Catalytic Applications". Chemical Reviews. 109 (5): 2061–2080. doi:10.1021/cr800270h. PMID 19326858.CS1 maint: uses authors parameter (link)
  2. Shin, Yeung-gyo K.; Szalda, David J.; Brunschwig, Bruce S.; Creutz, Carol; Sutin, Norman (1997). "Electronic and Molecular Structures of Pentaammineruthenium Pyridine and Benzonitrile Complexes as a Function of Oxidation State". Inorganic Chemistry. 36 (14): 3190–3197. doi:10.1021/ic9700967. PMID 11669976.
  3. Robert A. Heintz; Jennifer A. Smith; Paul S. Szalay; Amy Weisgerber; And Kim R. Dunbar (2002). "11. Homoleptic Transition Metal Acetonitrile Cations with Tetrafluoroborate or Trifluoromethanesulfonate Anions". Inorg. Synth. 33: 75–83. doi:10.1002/0471224502.ch2.
  4. Richard R. Thomas, Ayusman Sen (2007). Acetonitrile Complexes of Selected Transition Metal Cations. Inorg. Synth. Inorganic Syntheses. 28. pp. 63–67. doi:10.1002/9780470132593.ch14. ISBN 9780470132593.CS1 maint: uses authors parameter (link)
  5. I. Sotofte; R. G. Hazell; S. E. Rasmussen (1976). "Hexaacetonitrilenickel(II) Tetrachlorozincate. A Crystal Structure with Serious Overlap in the Patterson Function". Acta Crystallographica Section B. 32 (6): 1692–1696. doi:10.1107/S0567740876006249.
  6. Gregory J. Kubas; Lori Stepan van der Sluys (1990). Tricarbonyltris(Nitrile) Complexes of Cr, Mo, and W. Inorg. Synth. Inorganic Syntheses. 28. pp. 29–33. doi:10.1002/9780470132593.ch6. ISBN 9780470132593.
  7. Churchill, D.; Shin, J. H.; Hascall, T.; Hahn, J. M.; Bridgewater, B. M.; Parkin, G. (1999). "The Ansa Effect in Permethylmolybdenocene Chemistry: A [Me2Si] Ansa Bridge Promotes Intermolecular C−H and C−C Bond Activation". Organometallics. 18: 2403–2406. doi:10.1021/om990195n.
  8. García, J. J.; Arévalo, A.; Brunkan, N. M.; Jones, W. D. (2004). "Cleavage of Carbon−Carbon Bonds in Alkyl Cyanides Using Nickel(0)". Organometallics. 23 (16): 3997–4002. doi:10.1021/om049700t.CS1 maint: uses authors parameter (link)
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