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The characteristic chemistry of some organometallic nitrosyl complexes Martin, David Timothy


Treatment of a CH₂Cl₂ solution of (n⁵-C₅H₅ )W(NO)₂Cl with an equimolar amount of AgBF₄ results in the rapid formation of a precipitate of AgCl and a bright green solution of (n⁵-C₅H₅ )W(NO) ₂BF₄. This organometallic complex is a thermally unstable, air- and moisture-sensitve species whose characteristic chemistry is dominated by its electrophilic nature. Hence, it readily forms adducts of the type [(n⁵-C₅H₅ )W(NO) ₂L]BF₄ with Lewis bases, L, capable of good donation such as PPh₃ or P(OPh)₃. In an analogous manner, the novel complex [(n⁵-C₅H₅ )W(NO) (n²-C₈H₁₄ ) BF₄ results when (n⁵-C₅H₅ )W(NO) ₂BF₄ is exposed to an excess of cyclooctene. Other olefins, however, either do not react (e.g. Allylbenzene) or undergo dimerization (e.g. 1,1-diphenylethene) or isomerization (e.g. 2,3-dimethyl-1-butene), the latter transformations probably occurring via the formation of incipient carbocations. Phenylethyne is either polymerized or condensed with other unsaturated substrates (e.g. 2,3-dimethyl-2-butene). The organometallic electrophile also reacts with dichloromethane-soluble anionic nucleophiles, X to form (n⁵-C₅H₅ )W(NO) ₂X (X = H or Br) products. Of greater interest is the fact that (n⁵-C₅H₅ )W(NO) ₂BF₄ is sufficiently electrophilic to cleave the M-C linkages in organic derivatives of the main group elements (e.g. B, Al or Sn) and so produce some new (n⁵-C₅H₅ )W(NO) ₂R (R = C₂H₅, CH₂Ph or Ph) complexes. This chemistry has also been extended to encompass some of the pentamethylcyclopentadienyl analogues derivable from (n⁵-C₅Me₅ )W(NO) ₂BF₄. Treatment of (n⁵-C₅H₅ )M(NO) ₂CH₃ (M = Cr or Mo) with electrophiles such as HgCl₂, Ph₃C⁺, Ph₂CH⁺ and AlCl₃ results in rapid cleavage of the metal-methyl bond. Contrastingly, (n⁵-C₅H₅ )M(NO)₂ reacts much less readily with these reagents. These results point to a fundamental difference between the reactivity of these alkylnitrosyl complexes and their isoelectronic carbonyl analogues. Treatment of [(n⁵-C₅Me₅ )Mo(CO)₃CH₃ with an equimolar amount of HBF₄O(CH₃)₂ in CH₂Cl₂ results in the clean formation of [{(n⁵-C₅H₅ )Fe(CO)₂} ₂H]BF₄ which may be isolated in good yield. In contrast, 2 equivalents of the acid are required to consume completely [ (n⁵-C₅H₅ )Cr(NO)₂]₂, the principal organometallic product being (n⁵-C₅H₅ )Cr(NO)₂BF₄. This latter complex is not isolable, but it may be characterized spectroscopically and by its derivative chemistry. The propensities of the [(n⁵-C₅H₅ )M(LO)₂]₂, (M = Cr or Fe; L = N or C) dimers to undergo protonation or oxidative cleavage when treated with H+ are rationalized in terms of the initially formed [(n⁵-C₅H₅ )M(LO)₂]₂H⁺ adducts.

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