Reaction of the Tricyanoborate Dianion [B(CN)₃]^2– with HgCl₂

The very reactive [B­(CN)₃]^2– dianion has a strongly nucleophilic boron atom and can be used for the synthesis of tricyanoborates, which otherwise are difficult to access. Herein the reaction of this anion with HgCl₂ is reported. The main product is the anionic mercury complex [Hg­(B­(CN)₃)₂]^2–. Heteronuclear NMR spectroscopic experiments shows that the reaction proceeds via the intermediate [ClHgB­(CN)₃]^2–. Even though [Hg­(B­(CN)₃)₂]^2– is the main product, it is difficult to obtain it in pure form, because it slowly decomposes in the presence of water and air to [(NC)­HgB­(CN)₃]⁻. All three anions were fully characterized by hetereonuclear NMR spectroscopy (¹¹B, ¹³C, and ¹⁹⁹Hg). Single-crystal X-ray diffraction studies of the salts K­[ClHg­B­(CN)₃], [Ph₄P]₂[Hg­(B­(CN)₃)₂], K­[(NC)­Hg­B­(CN)₃], and [Ph₄P]­[(NC)­Hg­B­(CN)₃] revealed linear coordination environments around mercury for all anions. The Hg–B bonds range from 2.219(5) Å in [Hg­(B­(CN)₃)₂]^2– to 2.148(11) Å in [ClHgB­(CN)₃]⁻, are in accord with the sum of the covalent radii of mercury and boron, and can be described as covalent single bonds. A comparison with related complexes indicates that the [B­(CN)₃]^2– dianion is a stronger ligand than chloride, cyanide, or carbenes. [Hg­(B­(CN)₃)₂]^2– hydrolyses in solution only in the presence of oxygen. It is suggested that <i>cis</i>-[Hg­(OH)₂­(B­(CN)₃)₂]^2– is formed as a very unstable intermediate, which decomposes very fast to [(NC)­Hg­B­(CN)₃]⁻ and other products. The anion <i>cis</i>-[Hg­(OH)₂­(B­(CN)₃)₂]^2– would contain mercury in the unusual oxidation state +IV. Quantum-chemical calculations were performed to support this assumption