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)(-S)(3)((3)-S)(3)BH], the room temperature reaction of [Cp*NbCl4] (Cp* = (5)-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)(-S)(3)((3)-S)(3)(-S)BH], 1, and a hexa-sulfido cluster [(Cp*Nb)(3)(-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 (1)-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)(-Se)(4){-Se-2(Se-2)}], 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 H-1, B-11{H-1} and C-13{H-1} 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.