Base Stabilized σ ‐Borane Complex of Group 5 Metals: Synthesis and Structure of [( η 5 ‐C 5 Me 5 )Ta(H 3 BNC 5 H 4 ) (C5H 4 NS)( η 2 ‐S 2 )]

Azametallacyclopropane‐containing base stabilized borane complexes of group 5 transition metals have been synthesized and their structural aspects have been described. Treatment of Cp* based Ta and Nb chlorides, Cp*TaCl 4 and Cp*NbCl 4 with [LiBH 4 ⋅ THF] followed by addition of ligands, such as 2‐mercaptobenzothiazole, MBT, (C 7 H 5 NS 2 ) and 2‐mercaptobenzoxazole, MBO (C 7 H 5 NSO) led to the formation of complexes [Cp*M‐[BHS(CH 2 ENC 6 H 4 )(C 7 H 4 NSE)] ( 1 : M=Ta, E=S; 2 ; M=Nb, E=S; 3 : M=Ta, E=O; 4 ; M=Nb, E=O, Cp*=pentamethyl‐ η 5 ‐cyclopentadienyl). By means of UV‐vis absorption spectra, the electronic properties of these complexes associated with central metal atoms and heteroatoms (S or O) have been evaluated. In contrast, treatment of Cp*TaCl 4 with 2‐mercaptopyridine, MP, (C 5 H 5 NS) under the same reaction conditions yielded the agostic σ ‐borane Ta complex, [Cp*Ta(H 3 BNC 5 H 4 ) (C5H 4 NS)( η 2 ‐S 2 )], 5 . Unlike 1 – 4 , where the metals interact with boron through bridging sulphur, 5 shows a notable σ ‐B−H bond interaction with Ta. All spectroscopic data of 1 – 5 along with the X‐ray diffraction studies suggest complexes 2 , 4 , and 5 are base (amine) stabilized borane species. Computational studies based on Density Functional Theory (DFT) also supported this conclusion.