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

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posted on 20.04.2017, 13:49 by Eduard Bernhardt, Vera Bernhardt-Pitchougina, Carsten Jenne
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 cis-[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 cis-[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.