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 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.

The hexacyanodiborane(6) dianion [B2(CN)6](2-).

Diborane(6) dianions with substituents that are bonded to boron via carbon are very reactive and therefore only a few examples are known. Diborane(6) derivatives are the simplest catenated boron compounds with an electron-precise B-B σ-bond that are of fundamental interest and of relevance for material applications. The homoleptic hexacyanodiborane(6) dianion [B2 (CN)6 ](2-) that is chemically very robust is reported. The dianion is air-stable and resistant against boiling water and anhydrous hydrogen fluoride. Its salts are thermally highly stable, for example, decomposition of (H3 O)2 [B2 (CN)6 ] starts at 200 °C. The [B2 (CN)6 ](2-) dianion is readily accessible starting from 1) B(CN)3 (2-) and an oxidant, 2) [BF(CN)3 ](-) and a reductant, or 3) by the reaction of B(CN)3 (2-) with [BHal(CN)3 ](-) (Hal=F, Br). The latter reaction was found to proceed via a triply negatively charged transition state according to an SN 2 mechanism.

Convenient access to the tricyanoborate dianion B(CN)3²â» and selected reactions as a boron-centred nucleophile.

Alkali metal tricyanoborates M2B(CN)3 (M = Na, K) are accessible by the reaction of tricyanofluoroborates with alkali metals (i) in liquid NH3 or (ii) in THF-naphthalene. The M2B(CN)3 are versatile starting materials for the synthesis of K[RB(CN)3] (R = Et, C6F5, CH2=CHCH2).