The electronic structure of (η -Cp) Zr(NH -BB-NH ) (3 b) suggests that it could be a candidate for having a boron-boron triple bond in the cyclic system; however, computational studies shows that 3 b is a very high energy isomer on its potential energy surface. Replacement of amines with tricoordinate nucleophilic boron groups (η -Cp) Zr[B(PH ) -BB-B(PH ) ] (3 c) reduces the relative energy dramatically. The B≡B triple bond arises through the donation of two electrons from the metal fragment, ZrCp , to the in-plane π-bonding orbital of the central B-B unit. Strong σ-donating and chelating bis-phosphine ligands (Me P(CH ) PMe ), which stabilize donor-acceptor bonding interaction in gem-diborene L B-BBr (10), would be a good choice along the synthetic path towards 3 d, (η -Cp) Zr[B (Me P(CH ) PMe ) ]. A comparison of the energetics of the reaction leading to a cyclic boryne system (3 d), with the linear boryne isomer [(B NHC ) ] shows that the angle strain from cyclization is compensated by stabilization from the metal.

All the syntheses were carried out under an argon atmosphere with standard Schlenk line and glove box techniques. Compounds, such as, [Cp*TaCl4], [1] S2CPPh3, [2,3] and the external reference for the 11 B{ 1 H} NMR, [Bu4N][B3H8] [4] were synthesized according to the literature methods. Thin-layer chromatography was carried out on 250 mm aluminum supported silica gel TLC plates. NMR spectra were recorded in a 500 MHz Bruker FT-NMR spectrometer. Chemical shifts are referenced to (residual) solvent signals ( 1 H/ 13 C{ 1 H}; CDCl3: δ = 7.26/77.16 ppm) or external [Bu4N][B3H8] ( 11 B: δ = 30.07 ppm). Mass spectra were recorded in a Bruker Micro TOF-II mass spectrometer in ESI ionization mode. I.1 Synthesis and CharacterizationsScheme S1. Synthesis of compound 1. Synthesis of 1:A suspension of [Cp*TaCl4] (0.100 g, 0.22 mmol) in 8 mL toluene at -78 ºC was charged dropwise with lithium borohydride solution 2.0 M in THF (0.7 mL) in toluene (10 mL) over 15 min and kept under constant stirring for 1 hour. Then freshly prepared solution of excess S2CPPh3 (0.075 g of PPh3 dissolved in 6 mL of CS2) was added in the reaction mixture over 5 min and kept under constant stirring for 24h at room temperature.The colour of the reaction mixture changed from yellow to brown. The solvent was evaporated in vacuum; residue was extracted into hexane/CH2Cl2 (60:40 v/v) and passed through Celite. After the removal of the solvent from the filtrate, the residue was subjected to chromatographic workup using silica-gel TLC plates. Elution with hexane/CH2Cl2 (60:40 v/v) yielded orange [(Cp*Ta)2(µ,η 2 :η 2 -B2H5)(μ-H)(κ 2 ,µ-S2CH2)2], 1 (0.0125g, 14.02 %) along with known compound 2 (0.025 g, 33.30 %).

The reaction of [(Cp*Mo) (μ-Cl) B H ] (1) with CO at room temperature led to the formation of the highly fluxional species [{Cp*Mo(CO) } {μ-η :η -B H }] (2). Compound 2, to the best of our knowledge, is the first example of a bimetallic diborane(4) conforming to a singly bridged C structure. Theoretical studies show that 2 mimics the Cotton dimolybdenum-alkyne complex [{CpMo(CO) } C H ]. In an attempt to replace two hydrogen atoms of diborane(4) in 2 with a 2e [W(CO) ] fragment, [{Cp*Mo(CO) } B H W(CO) ] (3) was isolated upon treatment with [W(CO) ⋅thf]. Compound 3 shows the intriguing presence of [B H ] with a short B-B length of 1.624(4) Å. We isolated the tungsten analogues of 3, [{Cp*W(CO) } B H W(CO) ] (4) and [{Cp*W(CO) } B H Mo(CO) ] (5), which provided direct proof of the existence of the tungsten analogue of 2.

The synthesis and characterization of metal-free fluoroborylene [(Me-cAAC)2BF] (1) is reported.