Metal-Rich Metallaboranes: Synthesis, Structures and Bonding of Bi- and Trimetallic Open-Faced Cobaltaboranes

: 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 [LiBH 4 · THF] and subsequent photolysis with excess [BH 3 · THF] (THF = tetrahydrofuran) at room temperature afforded the 11-vertex tricobaltaborane nido -[(Cp*Co) 3 B 8 H 10 ] ( 1 , Cp* = η 5 -C 5 Me 5 ). The reaction of Li[BH 2 S 3 ] with the dicobaltaoctaborane(12) [(Cp*Co) 2 B 6 H 10 ] yielded the 10-vertex nido -2,4-[(Cp*Co) 2 B 8 H 12 ] 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 {CoB 3 } 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.


Introduction
Polyhedral cage expansion for the synthesis of large clusters has been the objective of boron-rich metallaborane and metallacarborane chemistry over the last six decades [1][2][3][4][5][6]. Molecular boron-rich clusters have found their place in many fields ranging from ceramics and polymers to boron neutron capture therapy and nanomaterials [7][8][9][10]. Those applications along with more fundamental studies of their specific and somewhat unique bonding and electronic structures have been the driving force towards the development of this chemistry [7,11,12]. In particular, following the pioneering work of Hawthorne et al. [13], several groups such as those of Evans [14], Welch [15], Xie [16,17], and ours [18][19][20] have contributed in enriching the library of the large 12-16 vertex single cage polymetalla(hetero)borane clusters over the years. The already extensive array of metallaborane clusters continues to grow in direct proportion with the advent of novel synthetic methods [9,11,17,[21][22][23][24][25][26][27][28][29][30][31][32][33]. However, it is difficult to come up with an integrated scheme aimed at the synthesis of polyhedra bearing definite geometry and composition. This is largely due to the undefined and uncontrolled nature of cluster growth from metal synthons or small preformed clusters. Even the most sought-after strategies like condensation involving monoborane reagents, insertion or fragmentation involving borane or metal carbonyl fragments and cluster fusion [34] rarely exhibit metallaboranes with a high metal−to−boron ratio [35][36][37][38][39].
Among the several strategies aimed towards cluster expansion that have been conceived over the years, the reaction of cyclopentadienyl halides or hydrides with monoborane reagents such as BH 3 ·THF (THF = tetrahydrofuran), [BHCl 2 ·SMe 2 ], or [LiBH 4 ·THF], is a strategy that has been thoroughly optimized [40,41] and has proven to be very useful for yielding a variety of higher vertex metallaborane clusters adopting unique polyhedral cage geometries and structural features [41][42][43][44][45]. Although, in most cases, the metallaboranes are generated in small yields with a low metal−to−boron ratio, it was the strategy that involved pyrolyzing [Cp*RhCl 2 ] 2 in large excess of [BH 3 ·THF], that allowed us to isolate 15and 16-vertex rhodaborane clusters [18]. Although fused clusters [46] or single cage clusters bearing 5-11 vertices are reported with the cobalt system [20,[46][47][48][49][50][51][52], isolation of analogous single cage supraicosahedral cobaltaboranes has not been successful up until now despite the fact that both rhodium and cobalt are group-9 metals. Indeed, our recent reports include the formation of a range of high-nuclearity cobaltaborane clusters, such as the hypoelectronic 9-vertex closo-[(Cp*Co) 2 4 ·THF] and subsequent thermolysis with 2-mercaptobenzothiazole in toluene [50]. Our renewed quest for higher nuclearity cobaltaboranes made us look for modified synthetic strategies and although the objective of isolating a single cage supraicosahedral cobaltaborane was not achieved, the treatment of [Cp*CoCl] 2 with [LiBH 4 ·THF] followed by photolysis with excess BH 3 ·THF afforded the 11-vertex-(hyper)nido-[(Cp*Co) 3 [53]. We have ourselves contributed significantly to this area by enriching the library of mono-and dicobaltadecaborane (14) analogues in the past [50][51][52]. Most recently, we have explored and reported the chemistry of dicobaltaoctaborane (12) with chalcogenated borohydride, Li[BH 2 E 3 ] [E = S, Se, or Te] which resulted in the characterization of novel chalcogenated analogues of dicobaltadecaborane (14) [54]. Careful analysis of the reaction between [(Cp*Co) 2 B 6 H 10 ] and Li[BH 2 S 3 ] led to the isolation of 2,4-[(Cp*Co) 2 B 8 H 12 ] (2), a new dicobaltadecaborane (14) analogue. The main results are presented in this article.

Reaction of (Cp*CoCl) 2 with [LiBH 4 ·THF] and [BH 3 ·THF]
The reaction of [Cp*CoCl] 2 with an eight-fold excess of [LiBH 4 ·THF] followed by photolysis with excess of BH 3 ·THF at room temperature for 16 h led to the formation of a green crystalline solid in 10% yield which was characterized as a novel open-faced 11-vertex cobaltaborane, (hyper)nido-[(Cp*Co) 3  Preparative thin-layer chromatography (TLC) allowed separation of 1 from the reaction mixture and hence enabled the characterisation of the pure compound. Multinuclear NMR (see Supplementary Materials Figures S1−S4 for spectra) and IR spectroscopies and solidstate structure determination by X-ray diffraction analysis helped establish the composition and structure of 1. A detailed account of the spectroscopic and structural characterizations of 1 follows.
[(Cp*Co) 3 B 8 H 10 ], 1. The room temperature 11 B{ 1 H} NMR of 1 shows the presence of four resonances between 71.4 and 19.7 ppm. The 1 H{ 11 B} NMR spectrum of 1 suggests the existence of two different Cp* environments appearing at 1.9 and 1.7 ppm with a relative ratio of 1:2. This is also corroborated by the 13 C{ 1 H} NMR spectrum. In addition to the chemical shifts corresponding to B-H t protons, the 1 H{ 11 B} NMR spectrum also shows a sharp upfield resonance in the hydride region at δ = −20.43 ppm which may typically be arising due to two equivalent Co-H-B protons. The infrared (IR) spectrum shows broad bands around 2438 cm −1 owing to stretching frequencies of the terminal B-H t bonds. Although the spectroscopic data indicated the formation of a large cluster, they were not sufficient to help conceptualize the actual identity of 1. A clear identification was obtained only after the crystal structure determination of 1 which was carried out with crystals obtained by slow evaporation of a concentrated hexane/CH 2 Cl 2 (90:10) solution of 1 kept at −10 • C.
Inorganics 2021, 9, x FOR PEER REVIEW 3 of 14 ratio of 1:2. This is also corroborated by the 13 C{ 1 H} NMR spectrum. In addition to the chemical shifts corresponding to B-Ht protons, the 1 H{ 11 B} NMR spectrum also shows a sharp upfield resonance in the hydride region at δ = −20.43 ppm which may typically be arising due to two equivalent Co-H-B protons. The infrared (IR) spectrum shows broad bands around 2438 cm −1 owing to stretching frequencies of the terminal B-Ht bonds. Although the spectroscopic data indicated the formation of a large cluster, they were not sufficient to help conceptualize the actual identity of 1. A clear identification was obtained only after the crystal structure determination of 1 which was carried out with crystals obtained by slow evaporation of a concentrated hexane/CH2Cl2 (90:10) solution of 1 kept at −10 °C.
The molecular structure of 1 ( Figure 1) adopts a classical 11-vertex nido core that can be formally generated by the removal of an apical vertex of the ubiquitous 12-vertex icosahedron ( Figure 2). The molecule has two minor disordered components with atoms B8 and B8′, each with 50% occupancy. Despite the lack of precision in some of the bond distances and angles of the cluster owing to the restraints set to resolve this disorder, the crystallographic data reasonably allow a clear determination of the structure. Only one of the residues (with B8′) is discussed and shown in Figure 1. The five atoms of the open face (Co1, B4, B5, B7, and B8′) of the cluster are four-connected vertices while the remaining six ones are all five-connected vertices ( Figure 1). This is in agreement with the NMR results discussed above which indicate that the higher chemical shifts in the 11 B NMR spectrum corresponds to the high connected B atoms with the chemical shifts for B1 and B2 accidentally eclipsing at 71.4 ppm thus displaying four resonances in the ratio 2:2:2:2. A crystallographic mirror plane passing through the atoms Co1, B1, and B2 bisects the molecule into two symmetrical entities, thereby rendering the chemical environments of Cp* ligands attached to Co2 and Co3 as equivalent.
The polyhedral cage geometry of cluster 1 looks reminiscent of that of classical 11vertex nido geometry of related clusters such as [C2B9H11] 2− , [(Cp*Rh)3B8H9(OH)3] [55], 8, [56], [8,8- [57], [9,9- [57], and [(μ-PPh2)(PPh3)2Pt2B9H6(O i Pr)3] [58] for instance which all possess 13 skeletal electron pairs (SEP), i.e., (n + 2) SEPs if n is the number of occupied vertices according to the Polyhedral Skeletal Electron Pair Theory (PSEPT) [59]. Interestingly, however, with only 12 skeletal electron pairs (SEPs), cluster 1 is two electrons short of the formal (n + 2) SEP count expected for nido clusters as required by the PSEPT [59]. As stated earlier, in addition to the BHt and Cp* protons, the 1 H{ 11 B} NMR spectrum shows the presence of only a single resonance for two bridging Co-H-B protons. The presence of two B-H-B protons which would have afforded a nido species with the "expected" electron count could not be traced from 1 H NMR. The molecular structure of 1 ( Figure 1) adopts a classical 11-vertex nido core that can be formally generated by the removal of an apical vertex of the ubiquitous 12-vertex icosahedron ( Figure 2). The molecule has two minor disordered components with atoms B8 and B8 , each with 50% occupancy. Despite the lack of precision in some of the bond distances and angles of the cluster owing to the restraints set to resolve this disorder, the crystallographic data reasonably allow a clear determination of the structure. Only one of the residues (with B8 ) is discussed and shown in Figure 1. The five atoms of the open face (Co1, B4, B5, B7, and B8 ) of the cluster are four-connected vertices while the remaining six ones are all five-connected vertices ( Figure 1). This is in agreement with the NMR results discussed above which indicate that the higher chemical shifts in the 11 B NMR spectrum corresponds to the high connected B atoms with the chemical shifts for B1 and B2 accidentally eclipsing at 71.4 ppm thus displaying four resonances in the ratio 2:2:2:2. A crystallographic mirror plane passing through the atoms Co1, B1, and B2 bisects the molecule into two symmetrical entities, thereby rendering the chemical environments of Cp* ligands attached to Co2 and Co3 as equivalent.
The polyhedral cage geometry of cluster 1 looks reminiscent of that of classical 11vertex nido geometry of related clusters such as [C 2 B 9 H 11 ] 2− , [(Cp*Rh) 3 [59]. Interestingly, however, with only 12 skeletal electron pairs (SEPs), cluster 1 is two electrons short of the formal (n + 2) SEP count expected for nido clusters as required by the PSEPT [59]. As stated earlier, in addition to the BH t and Cp* protons, the 1 H{ 11 B} NMR spectrum shows the presence of only a single resonance for two bridging Co-H-B protons. The presence of two B-H-B protons which would have afforded a nido species with the "expected" electron count could not be traced from 1 H NMR.
To the best of our knowledge cluster 1 is the first example of electron-poor 11vertex−12-SEP (hyper)nido species of this kind in the metallaborane cluster domain. Nevertheless, isoelectronic and isostructural examples are known in metallacarborane chemistry. Indeed, cluster 1 can be compared both electronically and structurally to some 11-vertex−12-SEP nido clusters such as [9,9-   To the best of our knowledge cluster 1 is the first example of electron-poor 11-vertex−12-SEP (hyper)nido species of this kind in the metallaborane cluster domain. Nevertheless, isoelectronic and isostructural examples are known in metallacarborane chemistry. Indeed, cluster 1 can be compared both electronically and structurally to some 11vertex−12-SEP nido clusters such as [9,9-   To the best of our knowledge cluster 1 is the first example of electron-poor 11-vertex−12-SEP (hyper)nido species of this kind in the metallaborane cluster domain. Nevertheless, isoelectronic and isostructural examples are known in metallacarborane chemistry. Indeed, cluster 1 can be compared both electronically and structurally to some 11vertex−12-SEP nido clusters such as [9,9-  Density functional theory (DFT) calculations were carried out at the B3LYP/LANL2DZ level of theory (see computational details in Section 3) on model compound 1 (Cp analogue of 1) to analyse the electronic structure and nature of bonding in the electron deficient molecule 1. Optimized bond distances in 1 are in accordance with those experimentally measured for the solid-state structure of 1 (Table S1). The molecular orbital (MO) analysis of 1 (Table S2) shows a large energy gap of 2.79 eV between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) in agreement with the electron count of 12 SEPs and the fact that only two bridging hydrogen atoms are present in the structure. The HOMO of 1 is mainly localized on the metal centres and the five-connected inner-core boron atoms, whereas the LUMO is predominantly localized on the metal centres with considerable participation of the boron atoms of the open face of the cluster ( Figure 4). We may wonder if cluster 1 could be reduced and reach the expected count of 13 SEPs to obey the PSEPT. It turns out that the LUMO + 1 is energetically close to the LUMO, separated only by 0.4 eV. This indicates that reduction of 1 is unlikely.  Density functional theory (DFT) calculations were carried out at the B3LYP/LANL2DZ level of theory (see computational details in Section 3) on model compound 1′ (Cp analogue of 1) to analyse the electronic structure and nature of bonding in the electron deficient molecule 1. Optimized bond distances in 1′ are in accordance with those experimentally measured for the solid-state structure of 1 (Table S1). The molecular orbital (MO) analysis of 1′ (Table S2) shows a large energy gap of 2.79 eV between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) in agreement with the electron count of 12 SEPs and the fact that only two bridging hydrogen atoms are present in the structure. The HOMO of 1′ is mainly localized on the metal centres and the five-connected inner-core boron atoms, whereas the LUMO is predominantly localized on the metal centres with considerable participation of the boron atoms of the open face of the cluster ( Figure 4). We may wonder if cluster 1 could be reduced and reach the expected count of 13 SEPs to obey the PSEPT. It turns out that the LUMO + 1 is energetically close to the LUMO, separated only by 0.4 eV. This indicates that reduction of 1 is unlikely.  Density functional theory (DFT) calculations were carried out at the B3LYP/LANL2DZ level of theory (see computational details in Section 3) on model compound 1′ (Cp analogue of 1) to analyse the electronic structure and nature of bonding in the electron deficient molecule 1. Optimized bond distances in 1′ are in accordance with those experimentally measured for the solid-state structure of 1 (Table S1). The molecular orbital (MO) analysis of 1′ (Table S2) shows a large energy gap of 2.79 eV between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) in agreement with the electron count of 12 SEPs and the fact that only two bridging hydrogen atoms are present in the structure. The HOMO of 1′ is mainly localized on the metal centres and the five-connected inner-core boron atoms, whereas the LUMO is predominantly localized on the metal centres with considerable participation of the boron atoms of the open face of the cluster ( Figure 4). We may wonder if cluster 1 could be reduced and reach the expected count of 13 SEPs to obey the PSEPT. It turns out that the LUMO + 1 is energetically close to the LUMO, separated only by 0.4 eV. This indicates that reduction of 1 is unlikely.
Dicobaltadecaborane (14) analogue, 2. Cluster 2 was isolated as a yellow solid in 15% yield. The mass spectrometric data for 2 showed molecular ion peaks at m/z 487.2692 which corresponds to the molecular formula [C20H42B8Co2] + . The 1 B{ 1 H} NMR showed chemical shifts at δ = 26.6, 11.4, and 5.6 ppm, whereas 1 H NMR showed a single chemical shift for 30 Cp* protons at δ = 1.69 ppm, and additionally, chemical shifts at δ = 4.38, 2.94, and 2.31 ppm which correspond to B-Ht protons. A distinct up-field resonance was observed at δ = −4.25 ppm which may be assigned to four equivalent B-H-B bridging protons. The chemical shifts displayed in the 13 C{ 1 H} NMR spectrum also corroborated with a single Cp* environment in the molecule. The infrared (IR) spectrum showed broad bands centered around 2493 cm −1 owing to stretching frequencies of the terminal B-H bonds.
Slow evaporation of a concentrated hexane/CH2Cl2 (80:20) solution of 2 kept at −10 °C yielded crystals suitable for X-ray diffraction analysis. The solid-state X-ray structure of 2 ( Figure 5), determined by single-crystal X-ray diffraction measurements clearly shows that it is a 10-vertex open nido-dicobaltaborane which is in absolute agreement with the spectroscopic as well as mass spectrometric data. It is apparent from its solid state X-ray structure that compound 2 adopts the same structural arrangement as the symmetric Scheme 2. Synthesis of a symmetric dicobaltaborane analogue of decaborane (14).
Dicobaltadecaborane (14) analogue, 2. Cluster 2 was isolated as a yellow solid in 15% yield. The mass spectrometric data for 2 showed molecular ion peaks at m/z 487.2692 which corresponds to the molecular formula [C 20 H 42 B 8 Co 2 ] + . The 1 B{ 1 H} NMR showed chemical shifts at δ = 26.6, 11.4, and 5.6 ppm, whereas 1 H NMR showed a single chemical shift for 30 Cp* protons at δ = 1.69 ppm, and additionally, chemical shifts at δ = 4.38, 2.94, and 2.31 ppm which correspond to B-H t protons. A distinct up-field resonance was observed at δ = −4.25 ppm which may be assigned to four equivalent B-H-B bridging protons. The chemical shifts displayed in the 13 C{ 1 H} NMR spectrum also corroborated with a single Cp* environment in the molecule. The infrared (IR) spectrum showed broad bands centered around 2493 cm −1 owing to stretching frequencies of the terminal B-H bonds.
Slow evaporation of a concentrated hexane/CH 2 Cl 2 (80:20) solution of 2 kept at −10 • C yielded crystals suitable for X-ray diffraction analysis. The solid-state X-ray structure of 2 ( Figure 5), determined by single-crystal X-ray diffraction measurements clearly shows that it is a 10-vertex open nido-dicobaltaborane which is in absolute agreement with the spectroscopic as well as mass spectrometric data. It is apparent from its solid state X-ray structure that compound 2 adopts the same structural arrangement as the symmetric dimetalla nido-2,4-[(Cp*M) 2 B 8 H 12 ] (M = Rh and Ir) [27,55] previously reported, analogues of decaborane (14). In the structure, a crystallographic mirror plane including the B6-B8 vector bisects the molecule into two equivalent {CoB 3 } units. As a result, the two Co atoms that reside on either side of the crystallographic mirror plane are chemically equivalent and hence correspond to the presence of single Cp* environment in the 1 H and 13 C{ 1 H} NMR spectra. The bridging hydrogen atoms lie on the open face of the sixmembered ring and their presence is confirmed both by 1 H NMR and X-ray diffraction studies. We noted that the average B-B bond length in 2 is somewhat shorter as compared to the Rh and Ir analogues [27,55] and follows the trend (Co < Rh < Ir) down the triad. A {Cp*Co} unit being isolobal with a BH unit, the cluster is isostructural and isoelectronic with nido-decaborane (14) which can be generated from the removal of the six-connected apical vertex of an octadecahedron (see bottom of Figure 3). Indeed, 2 can be regarded as a 10-vertex nido metallaborane cluster with 12 SEPs, as expected according to the PSEPT. 2 is then a new member of the series of ten-vertex−12-SEP polyhedral borane, carborane, or metallaborane clusters having the same SEP count and geometry. We have tried to compare its structural data and chemical shift values with a set of comparable cobaltaboranes with the formal electron count of 12 SEPs (Table 1). The unique feature of this reaction here (Scheme 2) is the insertion of two BH units in octaborane(12) that leads to cluster growth from 8 to 10 vertices. In order to acquire an insight into the structure and bonding of 2, we further did DFT calculations at the B3LYP/LANL2DZ level of theory (see computational details in Section 3 on model 2′ (Cp analogue of 2) as well as on its group-9 metallaborane model analogues 3′ (Cp analogue of 2,4-[(Cp*Rh)2B8H12] and 4′ (Cp analogue of 2,4-[(Cp*Ir)2B8H12]) for comparison. The optimized geometrical parameters obtained for 2′ compare fairly well with the corresponding crystallographically characterized data of 2 (Table S1). Additionally, the gauge-including atomic orbitals-density functional theory (GIAO-DFT calculated 11 B chemical shift values agree well with those observed experimentally ( Table 2) and are of sufficient accuracy to confirm assignments that are consistent with the empirical ones based simply on boron connectivity and environment (see above).
A molecular orbital analysis suggests a significant HOMO-LUMO energy gap (> 4 eV) for the three compounds in the order 2′ < 3′ < 4′ (Figure 6). Inspection of the nodal properties of the frontier MOs reveals that HOMOs weigh heavily on the respective metal centres, while the LUMOs are delocalized both on the metal centres and the five-connected inner-core boron atoms. Metal−metal Wiberg bond indices computed for 2′, 3′, and 4′ are very weak and close to each other (ca. 0.06), reflecting the nonappearance of bonding interaction between the metal centres in such species. In order to acquire an insight into the structure and bonding of 2, we further did DFT calculations at the B3LYP/LANL2DZ level of theory (see computational details in Section 3 on model 2′ (Cp analogue of 2) as well as on its group-9 metallaborane model analogues 3′ (Cp analogue of 2,4-[(Cp*Rh)2B8H12] and 4′ (Cp analogue of 2,4-[(Cp*Ir)2B8H12]) for comparison. The optimized geometrical parameters obtained for 2′ compare fairly well with the corresponding crystallographically characterized data of 2 (Table S1). Additionally, the gauge-including atomic orbitals-density functional theory (GIAO-DFT calculated 11 B chemical shift values agree well with those observed experimentally ( Table 2) and are of sufficient accuracy to confirm assignments that are consistent with the empirical ones based simply on boron connectivity and environment (see above).
A molecular orbital analysis suggests a significant HOMO-LUMO energy gap (> 4 eV) for the three compounds in the order 2′ < 3′ < 4′ (Figure 6). Inspection of the nodal properties of the frontier MOs reveals that HOMOs weigh heavily on the respective metal centres, while the LUMOs are delocalized both on the metal centres and the five-connected inner-core boron atoms. Metal−metal Wiberg bond indices computed for 2′, 3′, and 4′ are very weak and close to each other (ca. 0.06), reflecting the nonappearance of bonding interaction between the metal centres in such species. In order to acquire an insight into the structure and bonding of 2, we further did DFT calculations at the B3LYP/LANL2DZ level of theory (see computational details in Section 3 on model 2′ (Cp analogue of 2) as well as on its group-9 metallaborane model analogues 3′ (Cp analogue of 2,4-[(Cp*Rh)2B8H12] and 4′ (Cp analogue of 2,4-[(Cp*Ir)2B8H12]) for comparison. The optimized geometrical parameters obtained for 2′ compare fairly well with the corresponding crystallographically characterized data of 2 (Table S1). Additionally, the gauge-including atomic orbitals-density functional theory (GIAO-DFT calculated 11 B chemical shift values agree well with those observed experimentally ( Table 2) and are of sufficient accuracy to confirm assignments that are consistent with the empirical ones based simply on boron connectivity and environment (see above).
A molecular orbital analysis suggests a significant HOMO-LUMO energy gap (> 4 eV) for the three compounds in the order 2′ < 3′ < 4′ (Figure 6). Inspection of the nodal properties of the frontier MOs reveals that HOMOs weigh heavily on the respective metal centres, while the LUMOs are delocalized both on the metal centres and the five-connected inner-core boron atoms. Metal−metal Wiberg bond indices computed for 2′, 3′, and 4′ are very weak and close to each other (ca. 0.06), reflecting the nonappearance of bonding interaction between the metal centres in such species. In order to acquire an insight into the structure and bonding of 2, we further did DFT calculations at the B3LYP/LANL2DZ level of theory (see computational details in Section 3 on model 2′ (Cp analogue of 2) as well as on its group-9 metallaborane model analogues 3′ (Cp analogue of 2,4-[(Cp*Rh)2B8H12] and 4′ (Cp analogue of 2,4-[(Cp*Ir)2B8H12]) for comparison. The optimized geometrical parameters obtained for 2′ compare fairly well with the corresponding crystallographically characterized data of 2 (Table S1). Additionally, the gauge-including atomic orbitals-density functional theory (GIAO-DFT calculated 11 B chemical shift values agree well with those observed experimentally ( Table 2) and are of sufficient accuracy to confirm assignments that are consistent with the empirical ones based simply on boron connectivity and environment (see above).
A molecular orbital analysis suggests a significant HOMO-LUMO energy gap (> 4 eV) for the three compounds in the order 2′ < 3′ < 4′ (Figure 6). Inspection of the nodal properties of the frontier MOs reveals that HOMOs weigh heavily on the respective metal centres, while the LUMOs are delocalized both on the metal centres and the five-connected inner-core boron atoms. Metal−metal Wiberg bond indices computed for 2′, 3′, and 4′ are very weak and close to each other (ca. 0.06), reflecting the nonappearance of bonding interaction between the metal centres in such species. In order to acquire an insight into the structure and bonding of 2, we further did DFT calculations at the B3LYP/LANL2DZ level of theory (see computational details in Section 3 on model 2′ (Cp analogue of 2) as well as on its group-9 metallaborane model analogues 3′ (Cp analogue of 2,4-[(Cp*Rh)2B8H12] and 4′ (Cp analogue of 2,4-[(Cp*Ir)2B8H12]) for comparison. The optimized geometrical parameters obtained for 2′ compare fairly well with the corresponding crystallographically characterized data of 2 (Table S1). Additionally, the gauge-including atomic orbitals-density functional theory (GIAO-DFT calculated 11 B chemical shift values agree well with those observed experimentally ( Table 2) and are of sufficient accuracy to confirm assignments that are consistent with the empirical ones based simply on boron connectivity and environment (see above).
A molecular orbital analysis suggests a significant HOMO-LUMO energy gap (> 4 eV) for the three compounds in the order 2′ < 3′ < 4′ (Figure 6). Inspection of the nodal properties of the frontier MOs reveals that HOMOs weigh heavily on the respective metal centres, while the LUMOs are delocalized both on the metal centres and the five-connected inner-core boron atoms. Metal−metal Wiberg bond indices computed for 2′, 3′, and 4′ are very weak and close to each other (ca. 0.06), reflecting the nonappearance of bonding interaction between the metal centres in such species. In order to acquire an insight into the structure and bonding of 2, we further did DFT calculations at the B3LYP/LANL2DZ level of theory (see computational details in Section 3 on model 2′ (Cp analogue of 2) as well as on its group-9 metallaborane model analogues 3′ (Cp analogue of 2,4-[(Cp*Rh)2B8H12] and 4′ (Cp analogue of 2,4-[(Cp*Ir)2B8H12]) for comparison. The optimized geometrical parameters obtained for 2′ compare fairly well with the corresponding crystallographically characterized data of 2 (Table S1). Additionally, the gauge-including atomic orbitals-density functional theory (GIAO-DFT calculated 11 B chemical shift values agree well with those observed experimentally ( Table 2) and are of sufficient accuracy to confirm assignments that are consistent with the empirical ones based simply on boron connectivity and environment (see above).
A molecular orbital analysis suggests a significant HOMO-LUMO energy gap (> 4 eV) for the three compounds in the order 2′ < 3′ < 4′ (Figure 6). Inspection of the nodal properties of the frontier MOs reveals that HOMOs weigh heavily on the respective metal centres, while the LUMOs are delocalized both on the metal centres and the five-connected inner-core boron atoms. Metal−metal Wiberg bond indices computed for 2′, 3′, and 4′ are very weak and close to each other (ca. 0.06), reflecting the nonappearance of bonding interaction between the metal centres in such species. In order to acquire an insight into the structure and bonding of 2, we further did DFT calculations at the B3LYP/LANL2DZ level of theory (see computational details in Section 3 on model 2′ (Cp analogue of 2) as well as on its group-9 metallaborane model analogues 3′ (Cp analogue of 2,4-[(Cp*Rh)2B8H12] and 4′ (Cp analogue of 2,4-[(Cp*Ir)2B8H12]) for comparison. The optimized geometrical parameters obtained for 2′ compare fairly well with the corresponding crystallographically characterized data of 2 (Table S1). Additionally, the gauge-including atomic orbitals-density functional theory (GIAO-DFT calculated 11 B chemical shift values agree well with those observed experimentally ( Table 2) and are of sufficient accuracy to confirm assignments that are consistent with the empirical ones based simply on boron connectivity and environment (see above).
A molecular orbital analysis suggests a significant HOMO-LUMO energy gap (> 4 eV) for the three compounds in the order 2′ < 3′ < 4′ (Figure 6). Inspection of the nodal properties of the frontier MOs reveals that HOMOs weigh heavily on the respective metal centres, while the LUMOs are delocalized both on the metal centres and the five-connected inner-core boron atoms. Metal−metal Wiberg bond indices computed for 2′, 3′, and 4′ are very weak and close to each other (ca. 0.06), reflecting the nonappearance of bonding interaction between the metal centres in such species.  [27,55] previously reported, analogues of decaborane (14). In the structure, a crystallographic mirror plane including the B6-B8 vector bisects the molecule into two equivalent {CoB3} units. As a result, the two Co atoms that reside on either side of the crystallographic mirror plane are chemically equivalent and hence correspond to the presence of single Cp* environment in the 1 H and 13 C{ 1 H} NMR spectra. The bridging hydrogen atoms lie on the open face of the six-membered ring and their presence is confirmed both by 1 H NMR and X-ray diffraction studies. We noted that the average B-B bond length in 2 is somewhat shorter as compared to the Rh and Ir analogues [27,55] and follows the trend (Co < Rh < Ir) down the triad. A {Cp*Co} unit being isolobal with a BH unit, the cluster is isostructural and isoelectronic with nido-decaborane (14) which can be generated from the removal of the six-connected apical vertex of an octadecahedron (see bottom of Figure 3). Indeed, 2 can be regarded as a 10-vertex nido metallaborane cluster with 12 SEPs, as expected according to the PSEPT. 2 is then a new member of the series of ten-vertex−12-SEP polyhedral borane, carborane, or metallaborane clusters having the same SEP count and geometry. We have tried to compare its structural data and chemical shift values with a set of comparable cobaltaboranes with the formal electron count of 12 SEPs (Table 1). The unique feature of this reaction here (Scheme 2) is the insertion of two BH units in octaborane(12) that leads to cluster growth from 8 to 10 vertices.
.   In order to acquire an insight into the structure and bonding of 2, we further did DFT calculations at the B3LYP/LANL2DZ level of theory (see computational details in Section 3 on model 2 (Cp analogue of 2) as well as on its group-9 metallaborane model analogues  (Table S1). Additionally, the gauge-including atomic orbitals-density functional theory (GIAO-DFT calculated 11 B chemical shift values agree well with those observed experimentally ( Table 2) and are of sufficient accuracy to confirm assignments that are consistent with the empirical ones based simply on boron connectivity and environment (see above). A molecular orbital analysis suggests a significant HOMO-LUMO energy gap (>4 eV) for the three compounds in the order 2 < 3 < 4 ( Figure 6). Inspection of the nodal properties of the frontier MOs reveals that HOMOs weigh heavily on the respective metal centres, while the LUMOs are delocalized both on the metal centres and the five-connected inner-core boron atoms. Metal−metal Wiberg bond indices computed for 2 , 3 , and 4 are very weak and close to each other (ca. 0.06), reflecting the nonappearance of bonding interaction between the metal centres in such species.

General Methods and Instrumentation
All of the experiments were conducted under an argon atmosphere using standard Schlenk and glove-box techniques. Solvents were distilled prior to use under argon. Cp*H, CoCl2, n-BuLi in hexane, LiBH4 (2.0 M in THF), BH3•THF, and S powder were used as received (Sigma Aldrich, Bangalore, India). (Cp*CoCl)2 [67,68]

General Methods and Instrumentation
All of the experiments were conducted under an argon atmosphere using standard Schlenk and glove-box techniques. Solvents were distilled prior to use under argon. Cp*H, CoCl 2 , n-BuLi in hexane, LiBH 4 (2.0 M in THF), BH 3 ·THF, and S powder were used as received (Sigma Aldrich, Bangalore, India). (Cp*CoCl) 2 [67,68] [71] in benzene-d 6 (δ B , ppm, −30.07) was used as an external reference for 11 B{ 1 H} NMR spectra. The light source for the photoreaction described in this report was Oriel Instrument with a 500 W Hg/Xe arc lamp equipped with liquid and colored glass filters, irradiating at 280-780 nm.
Synthesis of 1: [Cp*CoCl] 2 (0.1 g, 0.22 mmol) was suspended in toluene (15 mL) in a flame-dried Schlenk tube and was cooled to −78 • C, [LiBH 4 ·THF] (0.6 mL, 1.20 mmol) was added to it and the reaction mixture was allowed to warm slowly to room temperature and was left to stir for another hour. [BH 3 ·THF] (3 mL) was added to the reaction mixture and the resultant mixture was slowly warmed to room temperature was allowed to stir for 30 min, and finally kept for photolysis at room temperature for 12 h. After completion of the reaction (monitored by TLC), the solvent was removed under vacuum and the residue was dissolved in hexane and passed through Celite. The solvent was evaporated under vacuum and the residue was purified using preparative silica-gel TLC plates by eluting with hexane/CH 2 Cl 2 (80:20) mixture that yielded green solid 1 (0.010 g, 10%) and known red solids [(Cp*Co) 2 B 6 H 10 ] (0.025 g, 24%), [(Cp*Co) 3 B 4 H 4 ] (0.018 g, 19%), and a few compounds which could not be isolated. The known cobaltaboranes were characterized with reference to their spectroscopic data reported earlier.

Computational Details
Gaussian 09 program package [76] was used for geometry optimizations of all model clusters. Computations were done with the Cp ligands in place of Cp* to reduce the computational effort (for optimized model clusters see Figures S10-S13). Gas-phase optimizations without any solvent effect or symmetry constraints, were done with the hybrid Becke-Lee-Yang-Parr (B3LYP) functional [77] in combination with the LANL2DZ (Los Alamos National Laboratory 2 Double-Zeta) basis set [78] for all the atoms from the EMSL (Environmental Molecular Sciences Laboratory) Basis Set Exchange Library [79]. Frequency calculations were carried out for all structures to check the nature of the stationary states and the absence of any imaginary frequency to confirm that the optimized geometries were genuine minima on the potential energy hypersurface. NMR chemical shifts were computed by employing the gauge-including atomic orbitals (GIAOs) method [80][81][82] using the optimized geometries at the B3LYP/LANL2DZ level. Chemical shifts corresponding to 11 B NMR were computed in relation to B 2 H 6 (B3LYP shielding constant for 11 B NMR: 83.6 ppm) and were then converted to the standard [BF 3 ·OEt 2 ] scale by adding 16.6 ppm (the experimental δ ( 11 B) value of B 2 H 6 ) to the computed values. 1 H NMR chemical shift calculations were done with TMS (SiMe 4 ) as the internal standard. Chemcraft [83] was used for generating orbital graphics and optimized structure plots. Natural Bond Orbital (NBO) analyses [84] allowed for arrival of the Wiberg Bond Indices (WBI) [85].

Conclusions
Results described in this article have demonstrated the scope of new methodologies for the synthesis of high-nuclearity cobaltaborane clusters. Firstly, one of the new routes described here has led to the isolation of a novel and unprecedented 11-vertex-12-SEP disobedient cobaltaborane cluster with a (hyper)nido M 3 B 8 core. The second route has allowed for synthesis and characterization of a classic example of sequentially generated higher nuclearity cluster with a 10-vertex-12-SEP nido core via cluster growth on a preformed template. Indeed, cobaltaboranes and cobaltaheteroboranes constitute a showcase for dynamic and unique structural features and as a result, we are always on the lookout for possibilities of generating novel sophisticated cobaltaborane frameworks. Amongst the several synthetic strategies that are recognized in this domain, the applicability of the ones reported here may be vital and might lead to the generation of sophisticated high-nuclearity metallaborane frameworks of other group-9 metals. The unusual formation of 1 and the open-faced geometries of both 1 and 2 should imply enhanced reactivity to such species. Further efforts are currently devoted to that purpose.
Author Contributions: K.P. and C.N. formulated and planned the experiments; K.P. and C.N. carried out the synthesis and spectroscopic analyses and discussed the results with S.G. and J.