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A study of group 6 and 8 metal sigma-carbonyl cations and related metal(II) hexafluoroantimonates(V) Sham, Iona Hiu Tung

Abstract

Predominantly a-bonded carbonyl ligands appear to cause large ligand field splittings and all Fe rj-carbonyl cations studied in this work are low spin. The Fe cation, [Fe(CO)e]²⁺ (structurally characterized [Sb2Fn]⁻ or [SbFe]⁻ salt), is diamagnetic. [Fe(CO)6][Sb2Fn]₂ was synthesized by oxidative carbonylation of Fe(CO)₅ with XeF2 in HF-SbFs and further purified by treatment of the crude product with F2 in FfF. This procedure was compared to previous methods of obtaining [Fe(CO)6][Sb2Fn]₂ using AsFs and CI2 as oxidizing agents in SbF5 as solvent. Magnetic susceptibility measurements were used to determine the amount of paramagnetic impurity, identified as Fe[SbFe]₂, in the samples. The thermolysis behaviours of [Fe(CO)6][Sb2Fii]₂ and [Fe(CO)6][SbF6]₂ were investigated using Differential Scanning Calorimetry. The formation of [Fe(CO)6][Sb2Fn]₂ and the by-product Fe[SbF6]2 via an intermediate of the form Fe(CO)4X2 (X = CI or F) was postulated. It was found that M(CO)₆ (M = Mo and W) undergoes facile 2-electron oxidation when treated with SbFs at 40 - 60°C. Cr(CO)₆ is likely to behave in the same way in SbFs at lower temperatures (20 - 25°C). For Mo and W, seven-coordinate carbonyl complexes, [{Mo(CO)4}₂(cis-u-F2SbF4)3]x[Sb2Fi,]x and [W(CO)₆(FSbF5)][Sb2Fii], were isolated and structurally characterized. For Cr, a transient carbonyl species was observed. The magnetic behaviours of metal(II) hexafluoroantimonates(V), M[SbFe]2 (M = Mn, Fe, Co, Ni, Cu and Pd), in general, are as predicted for metal ions in weak ligand fields of octahedral or pseudo-octahedral geometry. The experimental room temperature magnetic moments of these compounds are much closer to us, than μ[sub L+S] or μ[sub j]. This is an indication of quenching of orbital angular momentum by ligand fields, and shows that the metals in these systems cannot be modeled as "free" or "naked" metal ions. Addition reactions involving pyrazine (pyz) or 4,4'-bipyridine (4,4'-bipy) and several M[SbF6]₂ (M = Cr, M n , Fe, Co, Ni and Cu) salts yielded mixtures of [M(pyz)n]X₂ or [M(4,4'-bipy)n]X₂ complexes (X = F" and / or [SbF6]⁻); these complexes have different numbers, n, of pyz or 4,4' bipy ligands. The formation of pyz.2SbFs in the reactions involving pyz was observed. The crystal structure of pyz.2SbF5 shows the molecule to have D₂[sub h], symmetry.

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