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Transition metal triple‐decker complexes are an interesting class of sandwich complexes that engrossed great attention due to their structures and properties. Over the decades, synthesis of triple‐decker complexes featuring homocyclic, heterocyclic or π‐conjugated rings as middle decks have been abundantly reported. In this regard, the chemistry of such complexes bearing boron in the middle deck are well explored due to the ability of boron‐containing cycles to readily coordinate bifacially with metal atoms thereby forming triple‐decker complexes. On the other hand, electron counting rules and theoretical calculations have strengthened our knowledge of the structure and bonding in these complexes. Further, these complexes can be used as synthons to generate organometallic polymers having interesting electronic, optical and magnetic properties that can be appropriately tuned to cater to a wide range of applications. In our quest for novel metallaboranes and metallaheteroboranes, we have been successful in isolating various triple‐decker complexes that feature boron in the middle deck. This review explained elaborately the synthesis, structures, and bonding in such complexes reported by us and others. [ABSTRACT FROM AUTHOR]

The structure and bonding of two novel tungstaboranes which were synthesized using diverse synthetic methods are described. (i) The room-temperature photolysis of [Cp*W(CO)3Me] with [BH3·SMe2] led to the isolation of the hydrogen-rich tungstaborane [Cp*W(CO)2B3H8] (1). Its geometry consists of an arachno butterfly core similar to tetraborane(10) and obeys the Wade-Mingos electron counting rules (n vertices, n + 3 skeletal electron pairs (seps)). (ii) Further, the tungstaborane [(Cp*W)3(μ-H)2(μ3-H)(μ-CO)2B4H4] (4) was isolated by thermolysis reaction of a tungsten intermediate, obtained by low temperature reaction of [Cp*WCl4] and [LiBH4·THF] with [Cr(CO)5·THF]. Compound 4 which seems to have formed by replacement of a BH unit in [(Cp*W)2B5H9] by the isoelectronic fragment {Cp*W(CO)2}, adopts an oblato-nido hexagonal-bipyramidal core (n vertices, n–1 seps). Both compounds were characterized using multinuclear NMR, IR spectroscopy, mass spectrometry as well as single crystal X-ray diffraction analysis. In addition, density functional theory (DFT) calculations were performed in order to elucidate their bonding and electronic structures. [ABSTRACT FROM AUTHOR]

Metal-rich metallaboranes featuring bridging borylene and boride units of various transition metals. [Display omitted] • Metal-rich metallaborane clusters featuring triply/tetra bridging borylene and boride units are reviewed here. • Different structural motifs, bonding and chemistry of these clusters are presented. • Electron counts and spectroscopic and structural comparison have been discussed. Metal-rich metallaborane clusters, having triply and tetra bridging borylenes, and multicentred boride units are demanding renewed interest because of their diverse reactivity and unique structural features. Recent years have witnessed significant progress in the field of metal-rich metallaborane clusters. In spite of high instability of free borylene species, modern synthetic strategies utilizing transition metals have enabled to the isolation of borylene species, particularly as triply bridging borylene species. On the other hand, naked boron 'boride' has been encapsulated inside different types of transition metal and metallaborane clusters. The unique bonding ability of the boron of borylene and boride with multiple transition metals afforded a series of metal-rich metallaboranes. In this review, we have discussed the history of the synthesis of metal-rich clusters containing triply bridging borylene and boride moieties and their structure, bonding, and electron count. The reactivity of these metal-rich metallaboranes toward various substrates has also been discussed. [ABSTRACT FROM AUTHOR]

A metal rich cluster made of Groups 8 and 9 metals was synthesized featuring a µ 5 -boride moiety. [Display omitted] • A new type of μ 5 -boride cluster [(IrCp*){Ru(CO) 2 }{Ru(CO) 3 } 3 (µ -H) 5 B 2 H 2 (μ 5 -B)] (1) is synthesised. • Cluster 1 is a unique boride derived from an eight-vertex closo -dodecadeltahedron by removal of one five-connected vertex followed by capping of a Ru(CO) 3 moiety. • Trimetallic and tetrametallic metallaboranes with a µ 3 -coordinated triply-bridged borylene moiety is also isolated. A series of metal-rich clusters of group 9 containing boride or borylene units have been isolated and structurally characterized. The thermolysis reaction of arachno- [IrCp*H 2 (B 3 H 7)] ( I ) with [Ru 3 (CO) 12 ] allowed to isolate the µ 5 -boride nido -[(IrCp*){Ru(CO) 2 }{Ru(CO) 3 } 3 (µ -H) 5 B 2 H 2 (μ 5 -B)] (1) and the tetrametallic cluster [(IrCp*) 2 {Ru(CO) 2 }{Ru(CO) 3 }(μ -CO) 2 (μ 3 -BH)] (2) with a µ 3 -coordinated triply-bridged borylene moiety. The geometry of cluster 1 can be derived from an eight-vertex closo -dodecadeltahedron by removal of one five-connected vertex followed by capping of a Ru(CO) 3 moiety. The borylene moiety (BH) of cluster 2 is connected in a μ 3 -fashion to the deltahedral face of a tetrametallic tetrahedron (Ir 2 Ru 2). To synthesize the Mn analogue of 2 , thermolysis reaction with [Mn 2 (CO) 10 ] was carried out, that afforded the trimetallic tetrahedral borylene [(IrCp*) 2 {MnH (CO) 3 }(μ -CO) 2 (μ 3 -BH)] (3) along with the known face-fused boride [{Ir(CO) 2 } 3 (IrCp*) 3 (μ 3 -CO)(μ -CO)(μ 5 -BH)] (4). Clusters 2 and 3 are the first triply-bridged borylenes having an Ir metal atom in the core. All these clusters were characterized by NMR (1H, 11B and 13C), infrared (IR) spectroscopies and mass spectrometry. The structure of cluster 1 was confirmed by X-ray diffraction analysis. In addition, density functional theory (DFT) calculations were conducted to interpret and study the nature of bonding and electronic structure of cluster 1. [ABSTRACT FROM AUTHOR]

• We have synthesized a triple-decker complex of molybdenum with a planar middle-deck [(Cp*Mo) 2 { µ - η 6: η 6-B 4 H 4 Co 2 (CO) 5 }(H) 2 ], 1. Compound 1 is a rare example of 24-valence-electron (ve) triple-decker sandwich complex having bridging hydrogen atoms. • Further, we have synthesized an octahedral cluster of tungsten, [{Cp*W(CO) 2 } 2 (µ -H) 2 B 3 H 3 Co 2 (CO) 4 ] 2 , along with the formation of the bimetallic cluster [(Cp*W) 2 B 3 H 3 (µ -H) 2 Co 2 (CO) 4 (µ -CO) 2 ] 3. The cage geometry of compound 2 is based on an octahedron with one additional vertex capping a trigonal face. [Display omitted] Recent isolation and structural characterization of a planar triple-decker metallaborane complex of tungsten prompted us to synthesize its molybdenum analogue. Therefore, we have explored the thermolysis reaction of the intermediates obtained from the low-temperature reaction of [Cp*MoCl 4 ] (Cp* = η 5-C 5 Me 5) and LiBH 4 with Co 2 (CO) 8. The reaction indeed produced the analogous triple-decker complex of molybdenum with a planar middle-deck [(Cp*Mo) 2 { µ - η 6: η 6-B 4 H 4 Co 2 (CO) 5 }(H) 2 ], 1 , along with the formation of the reported [(Cp*Mo) 2 B 5 H 9 ] metallaborane. In an effort to isolate multi-decker complexes with a different geometries, we have also further explored the thermolysis reaction of the intermediates obtained from the low-temperature reaction of [Cp*WCl 4 ] and LiBH 4 with Co 2 (CO) 8. The reaction afforded an octahedral cluster, [{Cp*W(CO) 2 } 2 (µ -H) 2 B 3 H 3 Co 2 (CO) 4 ], 2 , along with the formation of the bimetallic cluster [(Cp*W) 2 B 3 H 3 (µ -H) 2 Co 2 (CO) 4 (µ -CO) 2 ], 3. The cage geometry of compound 2 is based on an octahedron with one additional vertex capping a trigonal face. That of compound 3 is a bicapped trigonal-bipyramidal analogue to the parent compound [(Cp*W) 2 B 5 H 9 ]. While compound 1 has been characterized by various spectroscopic analyses, compounds 2 and 3 have been characterized by multiple spectroscopic and single-crystal X-ray diffraction analyses. Density-functional theory (DFT) calculations account for their stability and structural variation. [ABSTRACT FROM AUTHOR]