The reaction of [os₃(μ-H)₂(CO)₉L] (L = CO or PEt₃) with dimethylcyanamide, Me₂NCN: X-ray crystal structure of [os₃(μ-H)(μ-NCHNMe₂)(CO)₁₀] and the reactions of this complex with acids

The reaction of [Os₃H₂(CO)₁₀] with Me₂NCN gives a 1:1 adduct, [Os₃H(μ-H)(NCNMe₂)(CO)₁₀] (1), which, on reflux in hexane solution, rearranges to give [OS₃(μ-H)(μ-NCHNMe₂)(CO)₁₀] (2). The complex [Os₃H₂(CO)₉(PEt₃)] reacts similarly to give [OS₃(μ-H)(μ-NCHNMe₂)(CO)₉(PEt ₃)] (3) which exists in solution as a mixture of two isomers which could not be separated by t.I.c. An X-ray diffraction study of complex (2) shows that both the NCHNMe₂ group and the hydride bridge the short edge [Os-Os 2.795(1) Å] of an isosceles Os₃ triangle. The ten carbonyl groups are all terminal, four bonded to the unique Os, and three to each of the Os atoms which are also co-ordinated to the bridging ligands. Complex (2) reacts with strong acids, HX (X = CF₃CO₂ or Cl), and, in the presence of HBF₄·R₂O (R = Me or Et), with weak acids (X = CH₃CO₂, MeO, HO, or HOCH₂CH₂O) to give complexes of the type [OS₃(μ-H)(μ-X)(CO)₁₀]. The mechanism of this reaction is discussed in the light of a study by ¹H n.m.r. spectroscopy of the protonation of (2) at low temperatures. Results of the thermolysis and attempted hydrogenation of complex (2) are also reported.