Metal carbonyl syntheses XXII. Low-pressure carbonylation of [MOCl₄]^- and [MO₄]^-. The technetium(I) and rhenium(I) complexes [NEt₄]₂[MCl₃(CO)₃]

Low pressure carbonylation (1 atm) of [MOCl₄]^- or [MO₄]^- in the presence of BH₃/THF and halides X^- results in the clean formation of [NEt₄]₂[MX₃(CO)₃] containing monovalent rhenium or technetium (M = Re, Tc, X = Cl, Br). In the case of the radioactive element technetium this low pressure synthesis is an important progress since the potential hazards of traditional high-pressure carbonylations are thus avoided. The complex [NEt₄]₂[ReBr₃(CO)₃] crystallizes in the space group P-1 with a = 10.166(2), b = 16.393(4), c = 17.243(5) A ̊ and α = 69.27(2), β = 86.42(2), γ = 88.61(2)°, Z = 4, and V = 2682(1) A ̊³. [NEt₄]₂[MX₃(CO)₃] is a versatile precursor compound of other Re(I) and Tc(I) complexes: substitution of the halide ligands by a variety of σ-donor ligands is facile even under mild conditions. Examples include reactions with Cn^tBu to yield quantitatively TcCl(CN^tBu)₂(CO)₃ and with a tetradentate phosphine ligand to yield the dinuclear complex [Re₂Br₂(CO)₆(tetraphos)]. The X-ray structures of both compounds were determined. Dissolution of [NEt₄]₂[MX₃(CO)₃] in water under aerobic conditions results in the unusual organometallic aqua cation [M(OH₂)₃(CO)₃]⁺, which species is stable in water for days (IR spectroscopy).