TRANSFORMATION OF (ISOPROPYLIDENE)-2,5-DICHLORO-1,2,5-THIADIBOROLANE INTO NIDO-2,4,5-THIADICARBAHEXABORANE(5) DERIVATIVES - FORMATION AND COMPUTATIONAL ELUCIDATION OF A THIADICARBANONABORANE(8) WITH A NI-9[IV+IV] CONFIGURATION

The (isopropylidene)-2,5-dichloro-1,2,5-thiadiborolane (5b), obtained from 3,4-bis(dichloroboryl)-2,5-dimethyl-2,4-hexadiene (4) and (Me3Si) (2)S, reacts with Li[RBH3] (R = H, C6H5, CsMe4H) to yield the correspo nding derivatives of the nido-4,5-diisopropyl-2,4,5-thiadicarbahexabor anes 2. Replacements of the chlorine atoms in 5b with two hydride, or with one hydride and one aryl (phenyl or duryl) group, followed by the hydroboration of the isopropylidene substituents with RBH2 (R = H, C6 H5, C6Me4H) lead to four nido-2,4,5-thiadicarbahexaboranes (2a-d) in l ow yields. Their composition follows from MS and NMR data; not refined X-ray structural data of 2d support the proposed skeletal structure. In addition to 2a, larger thiacarboranes were detected by GC/MS and B- 11 NMR. The structure of the nine-vertex cluster 6a could be identifie d by applying the ab initio/IGLO/NMR method. Geometry optimizations at the MP2(fc)/6-31G level rule out the C-s nido-SC2B6H8 isomer 7. The C-2v structure of 6 consists of a nine-vertex cluster with two elongat ed B-B distances. As this 22e cluster 6 represents an exception to the Wade rules, the nature of the bonding of nine-vertex clusters with di fferent electron counts is discussed.