The chemistry of group VIB organometallic nitrosyl complexes
Creators: name: Hames, Barry Wayne
Publisher: name: University of British Columbia
Year: 2010

description: The reaction of chromocene with nitrogen monoxide in a variety of organic solvents leads to the formation of CpCr(NO)₂ (n¹-C₅H₅) as the major product, as well as CpCr(NO)₂ (NO₂) and [CpCr(NO)₂]₂ as minor products. Their formation in these conversions can be rationalized in terms of the reactive intermediate CpCr(NO) ₂. The reaction of photochemically generated molybdenocene with nitrogen monoxide to produce CpMo (NO) ₂ (n¹-C₅H₅) is also described. Sodium dihydridobis(2-methoxyethoxy)aluminate, I, undergoes metathetical reactions in benzene with a variety of nitrosyl halide complexes. Thus treatment of CpCr(NO) ₂X (X = NO₃, NO₂, I, n¹-C₅H₅, or BF₄), CpMn(CO) (NO)I, CpCo(NO)I, and [CpMo(NO) I₂] ₂ with I in 1:1 stoichiometries affords the respective dimeric compounds [CpCr (NO) ₂]₂, [CpMn (CO) (NO) ] ₂, [CpCo(NO)] ₂ and [CpMo (NO) I] ₂ . These latter conversions probably proceed via thermally unstable hydrido complexes. The chromium dimer also results from the reaction [CpCr(NO)₂ (CO)]PF₆ with the aluminum reagent and reacts further with I to produce in low yields a mixture of Cp₂Cr₂ (NO) (NH₂), Cp₂Cr₂ (NO) ₂ (NH₂)₂, and Cp₂Cr₂ (NO) ₂~ (NH₂)(OH). In a similar manner, Fe(NO) ₃Cl is converted by I to Fe₂ (NO) ₄ (NH₂)₂. Reduction of [CpCr(NO) ₂]₂ with BH₃ and with LiEt₃BH produces the same three bimetallic products as when I serves as the reducing agent, and in comparably low yields. However, with LiEt₃BH as reductant the complexes CpCr(NO) ₂Et and Cp₂Cr₂ (NO) ₃ (EtNBEt₂) are also formed, reflecting unprecedented modes of reactivity of the hydridoborate. An x-ray crystallographic analysis of the new Cp₂Cr₂ (NO) ₃ (EtNBEt₂) complex has been performed. The most chemically interesting feature of the molecular structure is the novel EtNBEt₂ ligand which is coordinated via N in a symmetrical fashion to the two Cr atoms. The coordination environment around N is that of a distorted tetrahedron, but the N-B distance of 1.459(5) &amp; suggests some degree of multiple bonding between these two atoms. Such an inference is consistent with the stability of the complex and its spectroscopic properties. The preparation and characterization of several organometallic hydridonitrosyl complexes, i.e. CpMo(NO)₂H and [Cp₂M₁M₂ (NO) ⁴H] ⁺X⁻ (M₁ = M₂ = Mo, W; M₁ = Mo, M₂ = W: X = BF₄ and/or PF₆) are described. The monometallic hydride is prepared by reduction of CpMo(NO)₂Cl with I, while the homonuclear bimetallic cations are formed upon reaction of CpM(NO)₂H (M = Mo, W) with 0.5 equivalents of a hydride abstraction agent such as Ph₃C⁺X⁻ or C₇H₇BF₄ in CH₂Cl₂. The heteronuclear cation can be prepared by the reaction of CpMo(NO)₂Cl with AgBF₄ to produce CpMo(NO)₂ ⁺BF₄⁻, which may then be reacted with CpW(NO)₂H to yield the cation. Attempts to deprotonate these cations with a variety of bases result in cleavage of the metal-metal bond to yield the hydride and a monometallic cation of the type CpM(NO)₂(L)⁺ (where L is the base used). The failure of the attempted deprotonations led to an examination of the Lewis base properties of CpW(NO)₂H and of the Lewis acid properties of CpW(N0)₂⁺. Specifically, the interaction of CpW(NO)₂H with a variety of soft (i.e. Cr(CO)₅, W(CO)₅, (MeCp)Mn(CO)₂, HgCl₂, and CdCl₂), borderline (i.e. ZnCl₂), and hard (i.e. H+, AlCl₃, and BEt₃) Lewis acids is observed spectroscopically. The observed Lewis base characteristics of CpW(NO) ₂H are discussed in light of these results, and when combined with the Lewis acid properties of CpW(NO)₂+, it is possible to rationalize the failure to prepare the dimers [CpM(NO)₂]₂ (M = Mo, W) via the desired route.