The reaction of sodium hydro-tris(phenylthioimidazolyl)borate (NaTm^Ph) with silicon tetraiodide gives rise to the first crystallographically characterised molecular silicon compounds, [Si(Tm^Ph)₂] 2X (X = I^– and I₃^–), in which the silicon is found in a regular S₆ environment. The [Si(Tm^Ph)₂]²⁺ cation is subjected to analysis using DFT methods to explain why an S₆ coordination motif is preferred to an S₄ coordination motif.

A comparison is made between the steric influences of a range of zinc hydrotris(pyrazolyl)borates and zinc hydrotris(thioimidazolyl)borates ([Zn(Tp^R)Cl], [Zn(Tm^R)Cl]; R = Me, iPr, Ph, tBu) by using inverse cone angle analysis. The study combines the crystallographic analysis of [Zn(Tm^iPr)Cl] and [Zn(Tm^Ph)Cl] with the data previously deposited with the Cambridge Crystallographic Data Centre. Despite efforts to manipulate the reactive pocket around the metal centre in M(Tm^R) complexes, the incorporation of sterically confining substituents onto the framework seems to have a minimal effect at the metal centre unless the group attached is very large.

A series of methimazole-based soft scorpionate anions ([RTm^Me]^–, R = H, Ph, Me, nBu) bearing substitution at the bridgehead boron have been used to produce a series of germanium complexes of general formulae [Ge(RTm^Me)₂]I₂. Structural analyses of the germanium complexes by X-ray crystallography reveal that they all contain an octahedral S₆ coordination sphere. The scorpionate anions (as their Li or Na salts) and their germanium complexes have been studied by thermogravimetric analysis. This analysis suggests that the degradation pathway for the free scorpionate anions differs from that of the complexes. Both pathways involve the loss of a methimazole ring thereby supporting the view that cleavage of the boron–nitrogen bonds can occur under thermally aggressive conditions. As expected, the presence of the germanium alters the degradation profile of the anion. In contrast to the free anions, the four complexes all display a similar mechanism for degradation. Although the presence of the germanium enforces a conformational change in the anions, its presence does not significantly increase the stability of the boron–nitrogen bonds.

Soft scorpionates have thus far been seen mainly as a family of ligands. Their chemistry is extended here to the production of novel cationic macrocycles using dihaloalkanes. By replacing the dihaloalkanes with mild oxidising agents (NO⁺, I₂) we obtain two unique polycyclic heterocycles. The mechanism which leads to the formation of these polycyclic heterocycles is investigated using ab initio DFT calculations.

The alkylation reactions of soft scorpionates are reported. The hydrotris(S-alkyl-methimazolyl)borate dications (alkyl=methyl, allyl, benzyl), which were prepared by the reaction of Tm^Me anion and primary alkyl halides, have been isolated and structurally characterised. The reaction is, however, not universally successful. DFT analysis of these alkylation reactions (CS versus BH alkylation) indicates that the observed outcome is driven by kinetic factors. Extending the study to incorporate alternative imine thiones (mercaptobenzothiazole, bz; thiazoline, tz) led to the structural characterisation of di[aquo-μ-aquohydrotris(mercaptobenzothiazolyl)boratosodium], which contains sodium atoms in the κ³-S,S,S coordination mode. Alkylation of Na[Tbz] and Na[tzTtz] leads to decomposition resulting in the formation of the simple S-alkylated heterocycles. The analysis of the species involved in these reactions shows an inherent weakness in the BN bond in soft scorpionates, which has implications for their use in more advanced chemistry.

This review describes the development of the anionic soft scorpionate ligand hydrotris(methimazolyl)borate (Tm^Me), its N-alkyl and N-aryl congeners and draws comparisons where appropriate with its close relatives, the dihydrobis(methimazolyl)borates (Bm^R). Key features of the interaction of these species with metal centres, including bonding modes, electron-donor properties, M···H–B interactions, ligand degradation and metallaboratrane formation are addressed and a summary of the coordination chemistry of these species with both main group and transition metals is given. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)