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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]

Triply-bridging bis-{hydrido(borylene)} and bis-borylene species of groups 6, 8 and 9 transition metals are reported. Mild thermolysis of [Fe2(CO)9] with an in situ produced intermediate, generated from the low-temperature reaction of [Cp*WCl4] (Cp*=η5 -C5Me5) and [LiBH4·THF] afforded triply-bridging bis-{hydrido(borylene)}, [(μ3- BH)2H2{Cp*W(CO)2}2{Fe(CO)2}] (1) and bis-borylene, [(μ3- BH)2{Cp*W(CO)2}2{Fe(CO)3}] (2). The chemical bonding analyses of 1 show that the B H interactions in bis-{hydrido (borylene)} species is stronger as compared to the M H ones. Frontier molecular orbital analysis shows a significantly larger energy gap between the HOMO-LUMO for 2 as compared to 1. In an attempt to synthesize the ruthenium analogue of 1, a similar reaction has been performed with [Ru3(CO)12]. Although we failed to get the bis-{hydrido(borylene)} species, the reaction afforded triply-bridging bis-borylene species [(μ3- BH)2{WCp*(CO)2}2{Ru(CO)3}] (2’), an analogue of 2. In search for the isolation of bridging bis-borylene species of Rh, we have treated [Co2(CO)8] with nido-[(RhCp*)2(B3H7)], which afforded triply-bridging bis-borylene species [(μ3- BH)2(RhCp*)2Co2(CO)4(μ-CO)] (3). All the compounds have been characterized by means of single-crystal X-ray diffraction study; ¹ H, 11B, 13C NMR spectroscopy; IR spectroscopy and mass spectromet [ABSTRACT FROM AUTHOR]

Synthesis, isolation, and structural characterization of unique metal rich diamagnetic cobaltaborane clusters are reported. They were obtained from reactions of monoborane as well as modified borohydride reagents with cobalt sources. For example, the reaction of [Cp*CoCl]2 with [LiBH4·THF] and subsequent photolysis with excess [BH3·THF] (THF = tetrahydrofuran) at room temperature afforded the 11-vertex tricobaltaborane nido-[(Cp*Co)3B8H10] (1, Cp* = η5-C5Me5). The reaction of Li[BH2S3] with the dicobaltaoctaborane(12) [(Cp*Co)2B6H10] yielded the 10-vertex nido-2,4-[(Cp*Co)2B8H12] cluster (2), extending the library of dicobaltadecaborane(14) analogues. Although cluster 1 adopts a classical 11-vertex-nido-geometry with one cobalt center and four boron atoms forming the open pentagonal face, it disobeys the Polyhedral Skeletal Electron Pair Theory (PSEPT). Compound 2 adopts a perfectly symmetrical 10-vertex-nido framework with a plane of symmetry bisecting the basal boron plane resulting in two {CoB3} units bridged at the base by two boron atoms and possesses the expected electron count. Both compounds were characterized in solution by multinuclear NMR and IR spectroscopies and by mass spectrometry. Single-crystal X-ray diffraction analyses confirmed the structures of the compounds. Additionally, density functional theory (DFT) calculations were performed in order to study and interpret the nature of bonding and electronic structures of these complexes. [ABSTRACT FROM AUTHOR]

In an attempt to expand the library of M2B5 bicapped trigonal-bipyramidal clusters with different transition metals, we explored the chemistry of [Cp*WCl4] with metal carbonyls that enabled us to isolate a series of mixed-metal tungstaboranes with an M2{B4M'} {M = W; M' = Cr(CO)4, Mo(CO)4, W(CO)4} core. The reaction of in situ generated intermediate, obtained from the low temperature reaction of [Cp*WCl4] with an excess of [LiBH4·thf], followed by thermolysis with [M(CO)5·thf] (M = Cr, Mo and W) led to the isolation of the tungstaboranes [(Cp*W)2B4H8M(CO)4], 1–3 (1: M = Cr; 2: M = Mo; 3: M = W). In an attempt to replace one of the BH—vertices in M2B5 with other group metal carbonyls, we performed the reaction with [Fe2(CO)9] that led to the isolation of [(Cp*W)2B4H8Fe(CO)3], 4, where Fe(CO)3 replaces a {BH} core unit instead of the {BH} capped vertex. Further, the reaction of [Cp*MoCl4] and [Cr(CO)5·thf] yielded the mixed-metal molybdaborane cluster [(Cp*Mo)2B4H8Cr(CO)4], 5, thereby completing the series with the missing chromium analogue. With 56 cluster valence electrons (cve), all the compounds obey the cluster electron counting rules. Compounds 1–5 are analogues to the parent [(Cp*M)2B5H9] (M= Mo and W) that seem to have generated by the replacement of one {BH} vertex from [(Cp*W)2B5H9] or [(Cp*Mo)2B5H9] (in case of 5). All of the compounds have been characterized by various spectroscopic analyses and single crystal X-ray diffraction studies. [ABSTRACT FROM AUTHOR]

The thermolysis of arachno-1 [(Cp*Ru)2(B3H8)(CS2H)] in the presence of tellurium powder yielded a series of ruthenium trithia-borinane complexes: [(Cp*Ru)2(η1-S)(η1-CS){(CH2)2S3BH}] 2, [(Cp*Ru)2(η1-S)(η1-CS){(CH2)2S3B(SMe)}] 3, and [(Cp*Ru)2(η1-S)(η1-CS){(CH2)2S3BH}] 4. Compounds 2–4 were considered as ruthenium trithia-borinane complexes, where the central six-membered ring {C2BS3} adopted a boat conformation. Compounds 2–4 were similar to our recently reported ruthenium diborinane complex [(Cp*Ru){(η2-SCHS)CH2S2(BH2)2}]. Unlike diborinane, where the central six-membered ring {CB2S3} adopted a chair conformation, compounds 2–4 adopted a boat conformation. In an attempt to convert arachno-1 into a closo or nido cluster, we pyrolyzed it in toluene. Interestingly, the reaction led to the isolation of a capped butterfly cluster, [(Cp*Ru)2(B3H5)(CS2H2)] 5. All the compounds were characterized by 1H, 11B{1H}, and 13C{1H} NMR spectroscopy and mass spectrometry. The molecular structures of complexes 2, 3, and 5 were also determined by single-crystal X-ray diffraction analysis. [ABSTRACT FROM AUTHOR]

In an objective to synthesize metallaheteroboranes containing heavier chalcogen atoms, we performed the reaction of dimetallaborane analogues of pentaborane(9) with various chalcogen sources. As a result, the thermolysis of nido -[1,2-(Cp*RuH) 2 B 3 H 7 ], 1 (Cp* = η 5 -C 5 Me 5 ) with mixture of S and Se powder was carried out, that led to the isolation of half sandwich dimetallaheteroborane arachno -[(1,2-Cp*Ru) 2 B 2 H 6 S 2 ], 2 . On the other hand, the reaction of nido - 1 with diorganyldichalcogenide ligands, [Ph 2 E 2 ] afforded chalcogen bridged half sandwich complexes [(2,3-Cp*Ru) 2 B 3 H 6 (μ-η 1 -EPh)], 4a-c ( 4a : E = S; 4b : E = Se and 4c : E = Te). Compound 2 can be derived from a closo- snub disphenoid by removing a 5-connect vertex followed by the removal of a 3-connect vertex. Compound 4a – c can be described as nido -square pyramidal structures, isoelectronic and isostructural with nido - 1 . All the compounds have been characterized by mass spectrometry, 1 H, 11 B and 13 C spectroscopy. Further, the geometry of compounds 2 , 3 , 4b and 4c were unequivocally established by crystallographic analysis. [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]

To build upon our earlier results of heterometallic metallaboranes employing metal carbonyls, we performed the reaction of nido -[(Cp*Rh) 2 B 3 H 7 ] ( 1 ) ( nido - 1 ) with [M(CO) 5 ·THF] (M = Mo or W) that yielded the trimetallic metallaborane clusters [(Cp*Rh) 2 M(CO) 3 (μ-CO)(μ 3 -BH)(B 2 H 4 )] ( 3 : M = Mo; 4 : M = W) having a capped borylene fragment and trimetallic triply bridging borylene complexes [(Cp*Rh) 2 (μ 3 -BH)(μ-CO)M(CO) 5 ] ( 5 : M = Mo; 6 : M = W). The chemistry of trimetallic triply bridging borylene complexes ( 5 and 6 ) were explored with Lewis bases such as tert- butyl isocyanide and bisphosphine ligands. Photolysis of 5 and 6 with tert -butyl isocyanide yielded [(Cp*Rh) 2 (μ 3 -BH)(μ-CO)M(CO) 4 (CN- t Bu)] ( 7 : M = Mo; 8 : M = W) and with phosphines, PPh 2 (CH 2 ) n PPh 2 (n = 1, 2) they resulted in the formation of [(Cp*Rh) 2 (μ 3 -BH)(μ-CO)M(CO) 4 ((PPh 2 ) 2 (CH 2 ) n )] ( 9 : n = 1, M = Mo; 10 : n = 1, M = W; 11 : n = 2, M = Mo; 12 : n = 2, M = W). All the new compounds have been characterized in solution by mass spectrometry and NMR spectroscopic techniques. The structural aspects were unambiguously established by X-ray crystallographic analysis of 3 – 4 and 7 – 10 . [ABSTRACT FROM AUTHOR]

In an attempt to synthesize expanded-cage metallaheteroboranes containing heavier chalcogen atoms, the reaction of diruthenaborane analogue of pentaborane(9), nido -[1,2-(Cp*RuH) 2 B 3 H 7 ] ( 1 ) with phenyl-chalcogenoborates Li[BH 3 (EPh)] (E = S, Se or Te) was carried out. Thermolysis of nido - 1 with Li[BH 3 (SPh)] led to the formation of the dimetalla-pentaborane(11) analogue arachno -[(Cp*Ru) 2 B 3 H 8 (SPh)] ( 2 ). In parallel to the formation of 2 , the reaction also yielded three B-H functionalized compounds, namely [(Cp*Ru) 2 B 4 H 7 (Ph)] ( 3 ), [(Cp*Ru) 2 B 4 H 7 (Cl)] ( 4 ) and [(Cp*Ru) 2 B 4 H 6 (SPh)(Cl)] ( 5 ). On the other hand, reaction of 1 with Li[BH 3 (SePh)] led to the formation of the diruthenium analogue of hexaborane(10) nido -[(Cp*Ru) 2 B 4 H 9 (SePh)] ( 6 ), whereas Li[BH 3 (TePh)] yielded the capped nido -pentagonal-pyramidal [(Cp*Ru) 2 B 4 H 6 Te] ( 7 ). Compound 7 is a rare ruthenaborane cluster containing a heavier chalcogen element (Te). All the compounds were characterized by mass spectrometry and 1 H, 1 H{ 11 B}, 11 B{ 1 H} and 13 C{ 1 H}NMR spectroscopy. The solid state X-ray structures of all the compounds were unequivocally established by crystallographic analysis. Additionally, the electronic properties of compound 2 were analyzed. [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]

Room temperature photolysis of [Cp*Mo(CO) 3 Me], 1 with excess of [BH 3 ·THF] led to the isolation of hydrogen-rich arachno -[Cp*Mo(CO) 2 B 3 H 8 ], 2 in good yield. The geometry of arachno - 2 consists of a butterfly core similar to that of 7 skeletal electron pairs tetraborane(10). Further, the metal fragment addition reaction of arachno - 2 with [Fe 2 (CO) 9 ] yielded a nido -[Cp*Mo(CO) 2 B 3 H 6 Fe(CO) 3 ], 3 that plays a pivotal role in bringing a change in the geometry from arachno - 2 to nido - 3 . The reaction of arachno - 2 with [Ru 3 (CO) 12 ], however, yielded known metal carbonyl compound [Cp*Mo(CO) 3 ] 2 , 4 . All the new compounds have been characterized in solution by mass spectrometry, IR, 1 H, 11 B, 13 C NMR spectroscopy and the solid state X-ray structures of 2 and 3 were unequivocally established by X-ray diffraction analysis. Additionally, we have studied the bonding nature of compounds 2 and 3 with the help of density functional theory (DFT) calculations. [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]

Diborane(6), diborane(4) and diborene(2) complexes of various transition metals. • Diboranes are valuable compounds for numerous synthetic reactions. • The diborene(2) complexes having boron-boron double bond are sparsely explored unlike alkene complexes. • Diborane(4/6) and diborene(2) complexes stabilized in the coordination sphere of transition metals are reviewed here. • Various synthetic routes for the synthesis of diborane and diborene complexes are discussed. • Photophysical properties of diborene species have been also discussed here. Diborane(4) and diborane(6) molecules are demanding renewed interest due to their varied reactivity towards diverse substrates. The chemistry of molecules comprising electron-precise B-B bonds has witnessed swift developments in the recent years. In spite of the continuous interest and extensive efforts in the synthesis of diborane compounds, the formation of boron-boron bonds is still difficult and uncontrolled. On the other hand, the diborene molecules (R-B B-R′; R, R′ = H, phosphine, amine, NHC etc), are also of significant interest owing to their ability to regulate the property of biradicals by changing the substituents of the parent diborene(2), HB BH. In addition, transition metal diborene species exhibit fascinating photo-physical properties. In this review, we have delivered a background of B-B bond formation and a synopsis of the latest developments in the synthesis of boron-boron single and multiple bonds. The reactivity of diborane/diborene complexes towards various transition metals has also been discussed in detail. [ABSTRACT FROM AUTHOR]