Chalcogen stabilized trimetallic clusters: synthesis, structures, and bonding of [(Cp*M)₃(E)_6+m(BH)_n] (M = Nb or Ta; E = S or Se; m = 0 or 1 or 2; n = 0 or 1)

In an effort to isolate the chalcogen-rich niobium analogue of [(Cp*Ta)₃(μ-S)₃(μ₃-S)₃BH], the room temperature reaction of [Cp*NbCl₄] (Cp* = η⁵-C₅Me₅) with Li[BH₂S₃] was carried out. Although the objective of isolating the niobium analogue was not achieved, the reaction yielded a homocubane-type cluster [(Cp*Nb)₃(μ-S)₃(μ₃-S)₃(μ-S)BH], 1, and a hexa-sulfido cluster [(Cp*Nb)₃(μ-S)₆], 2. Cluster 1 is a notable example of a homocubane-type cluster in which one of the vertices of the homocubane is missing. Compound 1 may be considered as a hypo-electronic cluster with an electron count of 64 cve (cve = cluster valence electrons), whereas compound 2 shows the presence of two doubly bridging η¹-S around each Nb–Nb bond. On the other hand, the room temperature reaction of [Cp*TaCl₄] with selenaborate ligand, [LiBH₂Se₃], led to the formation of [(Cp*Ta)₃(μ-Se)₄{μ-Se₂(Se₂)}], 3. Compound 3 is one of the rarest examples having a Ta₃Se₆ core structure with a unique diselenide bridging fragment. The presence of a short Se–Se bond of this diselenide unit makes this molecule of further interest. All these compounds were characterized by ¹H, ¹¹B{¹H} and ¹³C{¹H} NMR spectroscopy, infrared spectroscopy, mass spectrometry, and single-crystal X-ray crystallography. Density functional theory (DFT) calculations were carried out to provide insight into the bonding and electronic structures of these chalcogen-rich trimetallic clusters.