Treatment of [Cp*RuCl2]2, 1, [(COD)IrCl]2, 2 or [(p‐cymene)RuCl2]2, 3 (Cp*=η5‐C5Me5, COD= 1,5‐cyclooctadiene and p‐cymene=η6‐iPrC6H4Me) with heterocyclic borate ligands [Na[(H3B)L], L1 and L2 (L1: L=amt, L2: L=mp; amt=2‐amino‐5‐mercapto‐1,3,4‐thiadiazole, mp=2‐mercaptopyridine) led to the formation of borate complexes having uncommon coordination. For example, complexes 1 and 2 on reaction with L1 and L2 afforded dihydridoborate species [LAM(μ‐H)2BHL] 4–6 (4: LA=Cp*, M=Ru, L=amt; 5: LA=Cp*, M=Ru, L=mp; 6: LA=COD, M=Ir, L=mp). On the other hand, treatment of 3 with L2 yielded cis‐ and trans‐bis(dihydridoborate) species, [Ru{(μ‐H)2BH(mp)}2], cis‐7 and trans‐7. The isolation and structural characterization of fac‐ and mer‐[Ru{(μ‐H)2BH(mp)}{(μ‐H)BH(mp)2}], 8 from the same reaction offered an insight into the behaviour of these dihydridoborate species in solution. Fascinatingly, despite having reduced natural charges on Ru centres both at cis‐and trans‐7, they underwent hydroboration reaction with alkynes that yielded both Markovnikov and anti‐Markovnikov addition products, 10 a–d.

Several dihydridoborate group 7 metal complexes have been synthesized and their structural aspects have been described from various N,S-, N,N-, and N,Ochelated borate species, such as Na[(H 3 B)mp] (mp = 2-mercaptopyridyl), Na[(H 3 B)amt] (amt = 2-amino-5-mercapto-1,3,4-thiadiazolyl), Na[(H 3 B)hp] (hp = 2hydroxypyridyl), Na[(H 2 B)bap] (bap = bis(2-aminopyridyl)), and Na[(H 2 B)bdap] (bdap = bis(2,6-diaminopyridyl)). Room temperature photolysis of [M 2 (CO) 10 ] (M = Mn or Re) with these borate species afforded dihydridoborate complexes [(CO) 3 M(μ-H) 2 BHL] 1

Synthesis and structural elucidation of a series of thiolate-bridged heterobimetallic MnCo complexes are described. Irradiation of [Mn 2 (CO) 10 ] in the presence of Li[BH 3 SPh] followed by room-temperature reaction with [Cp*CoCl] 2 (Cp* = η 5 -C 5 Me 5 ) afforded dithiolate-and hydride-bridged dinuclear heterobimetallic MnCo complex [{(Mn(CO) 3 }(μ-SPh) 2 (μ-H){CoCp*}] (1). The solid-state structure of 1 established that the two metal fragments, {Mn(CO) 3 } and {Cp*Co}, are linked by a Mn−Co bond. In addition to 1, the reaction also yielded half-sandwiched trithiolate-bridged dinuclear MnCo complex [{Mn(CO) 3 }(μ-SPh) 3 (CoCp*)] (2) and a dinuclear heterometal-coordinated diborane analogue [{Mn(CO) 3 }(μ-η 2 :η 2 -SBH 3 )-(μ-H)(CoCp*)] (3). To isolate the Se analogues of 1−3, a similar reaction was carried out in the presence of Li[BH 3 SePh] that led to the formation of complexes [{(Mn(CO) 3 }(μ-SePh) 2 (μ-H)(CoCp*)] ( 4), [{Mn(CO) 3 }(μ-SePh) 3 (CoCp*)] ( 5), and [{Mn(CO) 3 }(μ-η 2 :η 2 -SeBH 3 )(μ-H)(CoCp*)] (6). All of the complexes were characterized by employing multinuclear nuclear magnetic resonance and infrared spectroscopies as well as mass spectrometric techniques. Single-crystal X-ray diffraction analyses of complexes 1, 2, and 4 helped to establish the molecular formulations and structural integrity of these complexes. The bonding interactions present in these di-or trichalcogenate-bridged dinuclear heterobimetallic complexes were explicated computationally by density functional theory calculations that supported the {Mn−H−Co} and Mn−Co bonding interactions in 1 and 4. The inherent electronic properties of all of the complexes were demonstrated by ultraviolet−visible spectroscopy. Furthermore, the critical involvement of the bridging chalcogenato functionalities was probed via cyclic voltammetry and complementary spectroelectrochemical studies.

Metallaborane clusters and their related chemistry were initially developed as a variant of metal–ligand coordination chemistry much along the lines of organometallic chemistry. However, owing to the insight imparted by the electron counting rules and isolobal analogy principle, the interconnections between apparently distinct molecules like polyhedral boranes and transition metal clusters were established. This certainly defined the scope of the area and notable advances toward the improvement of synthetic routes were witnessed. The advent of improved routes for synthesis widened the purview further by extending the limits of attainable sizes and geometries of polyhedral metallaboranes from diborane analogs to polyhedral cages of up to 12‐vertices to supraicosahedral and condensed cage macro‐polyhedral clusters. Wade–Mingos rules and Jemmis mno rule helped at gaining perspective about the unique structure and bonding present in this class of compounds. This article provides a brief overview of the syntheses and structural aspects in the context of electron counting rules of the most noted structural motifs assumed by polyhedral metallaboranes.