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Structure determination of some di-(tertiary arsine) derivatives of metal carbonyls Roberts, Paul J.

Abstract

The structures of four di-(tertiary arsine) derivatives of metal carbonyls have been determined by a selection of direct, Patterson, and Fourier methods, applied to Mo-[formula omitted] diffractometer data. 1,2- bis(dimethylarsino) tetrafluorocyclobutene-tri-iron decacarbonyl, Me₂AsC=C(AsMe₂) CF₂CF₂ .Fe₃ (CO)₁₀, crystallizes in the monoclinic space group P2₁/c, with a = 11.60, b = 20.04, c = 22.11 Å, β = 93.7°, Z = 8 (two molecules per asymmetric unit). The structure was refined by least-squares procedures (a total of 74 atoms) to a final R of 0.09 for 2524 observed (of a total of 3234) reflections. The molecule is best described as a derivative of Fe₃(CO)₁₂, in which one terminal carbonyl group on each of the two equivalent iron atoms is replaced by an arsenic atom of the di-(tertiary arsine) ligand. The central Fe₃As₂ cluster is significantly bent in one of the molecules of the asymmetric unit, but is more nearly planar in the other molecule. The Fe-Fe bond distances in the iron triangle (2.53,2.65,2.65 Å) do not differ significantly from those in the parent compound. Crystals of bis(1,2- bis(dimethylarsino)tetrafluoro cyclobutene)triruthenium octacarbonyl,(Me₂AsC=C(AsMe₂)CF₂CF₂)₂. Ru₃(CO)₈, are orthorhombic, space group Pbcn, a = 9.07, b = 18.53, c = 21.81 Å, Z = 4 (one-half molecule per asymmetric unit). The structure was refined by least-squares procedures to a final R of 0.078 for 1507 observed (of a total of 1712) reflections. The molecule lies on a crystallographic two-fold axis, and is best described as a derivative of Ru₃(CO)₁₂ in which two carbonyls on one ruthenium and one carbonyl on each of the other ruthenium atoms are replaced by the arsenic atoms of the bidentate di-(tertiary arsine) ligands, in such a way that each ligand bridges two ruthenium atoms, and one Ru-Ru bond remains unbridged. This unbridged Ru-Ru bond (2.785 Å) is significantly shorter than the bridged ones (2.853 Å) and than those of the parent Ru₃(CO)₁₂ (average 2.848 Å). The skeletons of the di-(tertiary arsine) ligands do not deviate significantly from exact planarity, the plane of each ligand being twisted 18° with respect to the plane of the ruthenium triangle. The mean Ru-As bond length is 2.407 Å. 1,2- bis(dimethylarsino)tetrafluorocyclobutene-triruthenium decacarbony1, Me₂AsC=C(AsMe₂)CF₂CF₂.Ru₃(CO)₁₀, crystallizes in orthorhombic space group. P2₁2₁2₁, a = 8.594, b = 18.795, c = 16.69 Å, Z = 4. The structure was refined by least-squares procedures to a final R of 0.076 for 1828 observed (of a total of 2028) reflections. The compound is a derivative of Ru₃(CO)₁₂ in which one equatorial carbonyl group on each of two ruthenium atoms is replaced by an arsenic atom of the di-(tertiary arsine) ligand in such a way that the plane of the ligand is twisted 18° with respect to the plane of the ruthenium triangle. Ru-Ru bond distances are 2.831, 2.831, and 2.858 Å, the difference between the short and long bond lengths being statistically significant and explicable in terms of the bonding characteristics of the ligand. 1,2- bis(dimethylarsino)hexafluoropropanemolybdenum tetracarbonyl, Me₂AsCF(CF₃)CF₂AsMe₂.Mo(CO)₄, crystallizes in the monoclinic space group C2/c, a = 25.06, b = 13.27, c = 11.56 Å, 6 = 102.8°, Z = 8. The structure was refined by least squares methods to a final R of 0.073 for 1510 , observed (of a total of 1750) reflections. The molecule is derived from Mo(CO)₆by replacing two carbonyl groups with the arsenic atoms of the chelating di-(tertiary arsine) ligand. Two of the carbon-fluorine bond distances (mean 1.505 Å) are significantly longer than the others (mean 1.30 Å) and the distance between the carbon atoms of the ligand skeleton is remarkably short (1.40). The weighted mean Mo-As bond length is 2.572 Å.

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