Oblate deltahedra in dimetallaboranes: geometry and chemical bonding

Rule breaker boron clusters: a new class of hypoelectronic osmaborane clusters [(Cp*Os)2BnHn] (n = 6-10)

Since the pioneering electron counting rule for borane clusters was proposed by Wade, the structures and bonding of boron clusters and their derivatives have been elegantly rationalised. However, this rule and its modified versions faced problems explaining the electronic structures of less spherical deltahedra, unlike the core geometries of borate dianions [BnHn]2- (n = 6-12). Herein, we report the isolation of a series of osmaborane clusters [(Cp*Os)2BnHn], 1-5, (n = 6-10) by the thermolysis of an in situ generated intermediate, obtained from the rapid condensation of [Cp*OsBr2]2 and [LiBH4·THF], with [BH3·THF] or [BH3·SMe2]. Interestingly, all these clusters show unusual less spherical deltahedral shapes that can be generated from canonical [BnHn]2- (n = 8-12) shapes by doing diamond-square-diamond (DSD) rearrangements. The DSD rearrangements led to the generation of higher degree vertices, which are occupied by Os atoms and also generated Os-Os bonds in these clusters. Theoretical calculations revealed that these Cp*Os⋯OsCp* interactions in clusters 1-5 played a crucial role in their structural shape and electron count. These less spherical deltahedral clusters are rare, and most significantly, clusters 1-5 with (n-1) skeleton electron pairs (SEPs) do not follow Wade-Mingos electron counting rules and can be classified as hypoelectronic closo clusters.

Transition Metal Triple-decker Sandwich Complexes Containing Group 13 Elements

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.

16-Vertex oblato-hypho-titanaborane [(Cp*Ti)2B14H18]

Although Lipscomb predicted in 1977 that supra-icosahedral boron clusters would be viable, their synthesis has been impeded by the unavailability of appropriate synthetic methodologies. Herein, we report the first examples of the open 16-vertex oblato-hypho-titanaborane clusters [(Cp*Ti)2B14H17R] (1: R = H; 2: R = Me) having a non-Wadean 19-skeletal-electron-pair count. Interestingly, these clusters show a six-membered [Ti2B4] open face, which could lead to closo-19-vertex clusters.

Polyhedral Dicobaltadithiaboranes and Dicobaltdiselenaboranes as Examples of Bimetallic Nido Structures without Bridging Hydrogens

The geometries and energetics of the n-vertex polyhedral dicobaltadithiaboranes and dicobaltadiselenaboranes Cp2Co2E2Bn−4Hn−4 (E = S, Se; n = 8 to 12) have been investigated via the density functional theory. Most of the lowest-energy structures in these systems are generated from the (n + 1)-vertex most spherical closo deltahedra by removal of a single vertex, leading to a tetragonal, pentagonal, or hexagonal face depending on the degree of the vertex removed. In all of these low-energy structures, the chalcogen atoms are located at the vertices of the non-triangular face. Alternatively, the central polyhedron in most of the 12-vertex structures can be derived from a Co2E2B8 icosahedron with adjacent chalcogen (E) vertices by breaking the E–E edge and 1 or more E–B edges to create a hexagonal face. Examples of the arachno polyhedra with two tetragonal and/or pentagonal faces derived from the removal of two vertices from isocloso deltahedra were found among the set of lowest-energy Cp2Co2E2Bn−4Hn−4 (E = S, Se; n = 8 and 12) structures.

Trivalent Polyhedra as Duals of Borane Deltahedra: From Molecular Endohedral Germanium Clusters to the Smallest Fullerenes

The duals of the most spherical closo borane deltahedra having from 6 to 16 vertices form a series of homologous spherical trivalent polyhedra with even numbers of vertices from 8 to 28. This series of homologous polyhedra is found in endohedral clusters of the group 14 atoms such as the endohedral germanium cluster anions [M@Ge₁₀]^3- (M = Co, Fe) and [Ru@Ge₁₂]^3- The next members of this series have been predicted to be the lowest energy structures of the endohedral silicon clusters Cr@Si₁₄ and M@Si₁₆ (M = Zr, Hf). The largest members of this series correspond to the smallest fullerene polyhedra found in the endohedral fullerenes M@C₂₈ (M = Zr, Hf, Th, U). The duals of the oblate (flattened) ellipsoidal deltahedra found in the dirhenaboranes Cp*₂Re₂B_n-2H_n-2 (Cp* = η⁵-Me₅C₅; 8 ≤ n ≤ 12) are prolate (elongated) trivalent polyhedra as exemplified experimentally by the germanium cluster [Co₂@Ge₁₆]^4- containing an endohedral Co₂ unit.

Triplet Spin-State Capped Deltahedral Structures Rather than Singlet Spin-State Oblatocloso Structures as Energetically Favored Dimanganaborane Structures

Density functional studies show that the singlet spin-state flattened oblatocloso deltahedral structures found experimentally in the dimetallaboranes Cp*₂Re₂B_n-2H_n-2 (Cp* = Me₅C₅; n = 8-12) of the third row group 7 element rhenium are not favored for analogous dimetallaboranes Cp₂Mn₂B_n-2H_n-2 (n = 8-14) of its first row congener manganese. Instead, the energetically preferred structures for the dimanganaboranes are higher spin-state triplet and quintet spin-state structures. This appears to be related to the lower ligand field splittings in complexes of the first row transition-metal manganese relative to analogous complexes of the third row transition-metal rhenium. The lowest-energy Cp₂Mn₂B_n-2H_n-2 (n = 8-13) structures typically have a central MnB_n-2 closo deltahedron with one face capped by the second CpMn unit. However, for the 14-vertex Cp₂Mn₂B₁₂H₁₂ system the lowest-energy structures consist of B₁₂ icosahedra with faces capped by both CpMn units. The thermochemistry of cluster buildup reactions of the type Cp₂Mn₂B_n-2H_n-2 + BH → Cp₂Mn₂B_n-1H_n-1 suggests that the 11- and 13-vertex structures are likely to be favored products in reactions of cyclopentadienylmanganese derivatives with borane sources. The paramagnetism of the predicted triplet and quintet spin states for the lowest-energy dimanganaboranes Cp₂Mn₂B_n-2H_n-2 (n = 8-14) suggests possible applications in novel magnetic materials.

Polyhedral [M2B5] Metallaborane Clusters and Derivatives: An Overview of Their Structural Features and Chemical Bonding

A large number of metallaborane clusters and their derivatives with various structural arrangements are known. Among them, M2B5 clusters and derivatives constitute a significant class. Transition metals present in these species span from group 4 to group 7. Their structure can vary from oblatonido, oblatoarachno, to arachno type open structures. Many of these clusters appear to be hypoelectronic and are often considered as 'rule breakers' with respect to the classical Wade-Mingos electron counting rules. This is due to their unique highly oblate (flattened) deltahedral structures featuring a cross-cluster M-M interaction. Many theoretical calculations were performed to elucidate their electronic structure and chemical bonding properties. In this review, the synthesis, structure, and electronic aspects of the transition metal M2B5 clusters known in the literature are discussed. The chosen examples illustrate how, in synergy with experiments, computational results can provide additional valuable information to better understand the electronic properties and electronic requirements which govern their architecture and thermodynamic stability.

Some interesting features of the rich chemistry around electron-deficient systems

In this short review, different phenomena that are triggered by the interaction of different compounds or clusters of compounds with electron-deficient systems, in particular beryllium and boron compounds, have been discussed in some detail. Particular attention was devoted to the huge acidity enhancements that can be induced through the interaction of conventional bases with B or Be containing compounds, which change these conventional bases in extremely strong proton donors. We have paid also attention to the cooperativity between Be bonds with other weak interactions, which results in a substantial increase of their strength, that can lead in some specific cases to the spontaneous formation of ion-pairs in the gas phase. Finally, the behavior of different Be derivatives as electron and anion sponges is discussed as well as the conditions needed to have clusters exhibiting rather strong Be–Be bonds, even though the Be–Be interaction in Be2 dimer is extremely weak. Finally, some attention was paid to systems with extremely short Be–Be distances but without a bond.

Deviations from the Most Spherical Deltahedra in Rhenatricarbaboranes Having 2n + 2 Wadean Skeletal Electrons

Density functional theory studies on the rhenatricarbaboranes C₃B_n-4H_n-1Re(CO)₃ (n = 7-12) show that the lowest energy polyhedra for n-vertex metallaboranes having 2n + 2 skeletal electrons and sufficiently dissimilar vertex atoms can deviate from the most spherical closo deltahedra predicted by application of the Wade-Mingos rules. Furthermore, the lowest energy structures of these rhenatricarbaboranes are found to avoid C-C edges and have carbon atoms located at degree 4 rather than degree 5 vertices. The lowest energy structures for the 7-vertex C₃B₃H₆Re(CO)₃ system all have a central C₃B₃Re closo deltahedron, namely the pentagonal bipyramid with the rhenium atom at a degree 5 axial vertex and all three carbon atoms at degree 4 equatorial vertices. However, the lowest energy structure for the 8-vertex C₃B₄H₇Re(CO)₃ is not the most spherical closo 8-vertex deltahedron, namely the bisdisphenoid, but instead a central C₃B₄Re hexagonal bipyramid with mutually nonadjacent degree 4 vertices for the carbon atoms. Similarly, the lowest energy 10-vertex C₃B₆H₉Re(CO)₃ structures are derived from isocloso deltahedra having three degree 4 vertices for all three carbon atoms rather than from the most spherical 10-vertex closo deltahedron, namely the bicapped square antiprism with only two degree 4 vertices. However, for the 9-vertex C₃B₅H₈Re(CO)₃ system, the most spherical closo deltahedron, namely the tricapped trigonal prism, has three mutually nonadjacent degree 4 vertices, which is ideal for the three carbon atoms and thus is the preferred deltahedron. The preferred deltahedron for the 11-vertex C₃B₇H₁₀Re(CO)₃ remains the most spherical closo deltahedron despite having only two degree 4 vertices for the carbon atoms. Furthermore, the six lowest energy 12-vertex C₃B₈H₁₁Re(CO)₃ structures are all based on the regular icosahedron generally favored in polyhedral borane chemistry despite its complete lack of degree 4 vertices for the carbon atoms.

Metal–metal bonding in deltahedral dimetallaboranes and trimetallaboranes: a density functional theory study

The skeletal bonding topology as well as the Re=Re distances and Wiberg bond indices in the experimentally known oblatocloso dirhenaboranes Cp*2Re2B n−2H n−2 (Cp*=η5Me5C5, n=8–12) suggest formal Re=Re double bonds through the center of a flattened Re2B n−2 deltahedron. Removal of a boron vertex from these oblatocloso structures leads to oblatonido structures such as Cp2W2B5H9 and Cp2W2B6H10. Similar removal of two boron vertices from the Cp2Re2B n−2H n−2 (n=8–12) structures generates oblatoarachno structures such as Cp2Re2B4H8 and Cp2Re2B7H11. Higher energy Cp2Re2B n−2H n−2 (Cp=η5-C5H5, n=8–12) structures exhibit closo deltahedral structures similar to the deltahedral borane dianions B n H n 2−. The rhenium atoms in these structures are located at adjacent vertices with ultrashort Re≣Re distances similar to the formal quadruple bond found in Re2Cl8 2− by X-ray crystallography. Such surface Re≣Re quadruple bonds are found in the lowest energy PnRe2B n−2H n−2 structures (Pn=η5,η5-pentalene) in which the pentalene ligand forces the rhenium atoms to occupy adjacent deltahedral vertices. The low-energy structures of the tritungstaboranes Cp3W3(H)B n−3H n−3 (n=5–12), related to the experimentally known Cp*3W3(H)B8H8, have central W3B n−3 deltahedra with imbedded bonded W3 triangles. Similar structures are found for the isoelectronic trirhenaboranes Cp3Re3B n−3H n−3. The metal atoms are located at degree 6 and 7 vertices in regions of relatively low surface curvature whereas the boron atoms are located at degree 3–5 vertices in regions of relatively high surface curvature. The five lowest-energy structures for the 11-vertex tritungstaborane Cp3W3(H)B8H8 all have the same central W3B8 deltahedron and differ only by the location of the “extra” hydrogen atom. The isosceles W3 triangles in these structures have two long ~3.0 Å W–W edges through the inside of the deltahedron with the third shorter W–W edge of ~2.7 to ~2.8 Å corresponding to a surface deltahedral edge.

Novel non-spherical deltahedra in tritungstaboranes related to the experimentally known Cp*3W3(H)B8H8

Low-energy Cp₃W₃(H)B_n−3H_n−3 (Cp = η⁵-C₅H₅; n = 5 to 12) structures have central W₃B_n−3 deltahedra with superimposed bonded W₃ triangles. Five lowest-energy Cp₃W₃(H)B₈H₈ structures all have the same central W₃B₈ deltahedron as found in the experimental Cp*₃W₃(H)B₈H₈ (Cp* = η⁵-Me₅C₅) structure.

Dimetallaborane analogues of the octaboranes of the type Cp2M2B6H10: structural variations with changes in the skeletal electron count

The structures and energetics of the complete series of hydrogen-rich dimetallaboranes Cp2M2B6H10 and Cp*2M2B6H10 (Cp = η(5)-C5H5; Cp* = η(5)-Me5C5; M = Pd, Pt; Rh, Ir; Ru, Os; Re; Mo, W; Ta), including the experimentally known Cp*2Rh2B6H10 and Cp*2W2B6H10 (Cp* = η(5)-Me5C5), have been investigated by density functional theory. The lowest energy structures of the hyperelectronic Cp2M2B6H10 (M = Pd, Pt; Rh, Ir) systems have central M2B6 frameworks with a hexagonal open face similar to the B8 networks in arachno-B8H14 and nido-B8H12. The two lowest energy structures for Cp2Rh2B6H10 and Cp*2Rh2B6H10, lying within 1 kcal mol(-1) of energy, differ only in the locations of the bridging hydrogen atoms around the hexagonal hole consistent with the experimentally observed fluxionality of the hydrogen atoms in Cp*2Rh2B6H10. Most of the lowest energy Cp2M2B6H10 (M = Ru, Os) structures also have a central M2B6 framework similar to B8H12, typically with such additional features as an additional metal-metal bond or a formal metal-metal double bond. A common motif for the low-energy structures of the hypoelectronic Cp2M2B6H10 (M = Re; Mo, W; Ta) systems, including the experimentally known Cp*2W2B6H10, is a central M2B4 octahedron with its two M2B faces capped by the remaining boron atoms and with four M-B edges bridged by hydrogen atoms. Such structures can also be considered as oblatonido structures derived from the experimentally known 9-vertex oblatocloso Cp*2Re2B7H7 structure by removal of the unique degree 4 vertex atom.

Hypo-electronic triple-decker sandwich complexes: synthesis and structural characterization of [(Cp*Mo)2{μ–η6:η6-B4H4E-Ru(CO)3}] (E = S, Se, Te or Ru(CO)3 and Cp* = η5-C5Me5)

Electron-poor triple-decker complexes, [(Cp*Mo)₂{μ–η⁶:η⁶-B₄H₄ERu(CO)₃}] (E = S, Se, Te, Ru(CO)₃) with hexagonal bridging ring composed of boron, ruthenium and chalcogen atoms.

Hydrogen migration in hypoelectronic biicosahedral metallaborane structures

Hydrogen migration occurs in the low-energy structures of the hypoelectronic systems CpMB₂₀H₁₇ (M = Fe, Ru, Os, Mo, W) to give structures having one or two M–B biicosahedral edges bridged by hydrogen atoms.

Novel non-spherical deltahedra in trirhenaborane structures

Low-energy Cp₃Re₃B_n−3H_n−3 (7 ≤ n ≤ 12) structures are found to be Re₃B_n−3 deltahedra with internally bonded Re₃ triangles. The rhenium atoms are generally located at degree 6 to 8 vertices and the boron atoms at degree 3 to 5 vertices. Low-energy Cp₃Re₃B₂H₂ and Cp₃Re₃B₃H₃ structures are found to be trigonal bipyramids and bicapped tetrahedra, respectively.

An electron-poor di-molybdenum triple-decker with a puckered [B4Ru2] bridging ring is an oblato-closo cluster

An unprecedented, 22-valence-electron triple-decker sandwich complex [(Cp*Mo)2{μ-η(6):η(6)-B4H4Ru2(CO)6}], 2, has been prepared. In an effort to generate analogous triple-deckers with group 6 metal carbonyl fragments in the middle deck, we have isolated [(Cp*MoCO)2(μ-H)2B4H4], 3, that provides the first direct evidence for the missing link between [(Cp*MoCl)2B3H7] and [(Cp*Mo)2B5H9] clusters.

Limited occurrence of isocloso deltahedra with 9 to 12 vertices in low-energy hypoelectronic diferradicarbaborane structures

Theoretical studies show that the 10-vertex system Cp(2)Fe(2)C(2)B(6)H(8) is the only one of the 2n skeletal electron Cp(2)Fe(2)C(2)B(n-4)H(n-2) systems (n = 9, 10, 11, 12) for which a true isocloso deltahedron having a single degree 6 vertex is highly favored over alternative structures. This is demonstrated by the occurrence of only the 10-vertex isocloso deltahedron as the central Fe(2)C(2)B(6) polyhedron in all nine of the Cp(2)Fe(2)C(2)B(6)H(8) structures within 8 kcal/mol of the global minimum. Low energy isocloso structures are also observed for the 11-vertex Cp(2)Fe(2)C(2)B(7)H(9). However, interspersed with these isocloso structures are Cp(2)Fe(2)C(2)B(7)H(9) structures based on deltahedra having two or more degree 6 vertices. For the 12-vertex Cp(2)Fe(2)C(2)B(8)H(10), the six lowest energy structures all have central Fe(2)C(2)B(8) deltahedra with two degree 6 vertices, one for each iron atom. The Cp(2)Fe(2)C(2)B(8)H(10) structures having a central Fe(2)C(2)B(8) icosahedron with all degree 5 vertices lie at significantly higher energies, starting at 17.8 kcal/mol above the global minimum. The 9-vertex Cp(2)Fe(2)C(2)B(5)H(7) system appears to be too small for isocloso structures to be favorable, although three such structures are found at energies between 5.5 and 8.0 kcal/mol above the global minimum. Five Cp(2)Fe(2)C(2)B(5)H(7) structures based on the tricapped trigonal prism lie in an energy below the lowest energy isocloso structure. The lowest energy Cp(2)Fe(2)C(2)B(5)H(7) structure and two higher energy structures within 8.0 kcal/mol of the global minimum have central Fe(2)C(2)B(5) deltahedra with a degree 6 vertex for each iron atom.

Closo versus hypercloso metallaboranes: A DFT study

The structures and electronic relationship of 9-, 10-, 11-, and 12-vertex closo and hypercloso (isocloso) metallaboranes are explored using DFT calculations. The role of the transition metal in stabilizing the hypercloso borane structures is explained using the concept of orbital compatibility. The hypercloso structures, C(6)H(6)MB(n-1)H(n-1) (n = 9-12; M = Fe, Ru, and Os) are taken as model complexes. Calculations on metal free polyhedral borane B(n)H(n) suggest that n vertex hypercloso structures need only n skeleton electron pairs (SEPs), but the structure will have one or more six-degree vertices, whereas the corresponding closo structures with n + 1 SEPs have only four- and five-degree vertices. This high-degree vertex of hypercloso structures can be effectively occupied by transition metal fragments with their highly diffused orbitals. Calculations further show that a heavy transition metal with more diffused orbitals prefers over a light transition metal to form hypercloso geometry. This is in accordance with the fact that there are more experimentally characterized hypercloso structures with the heavy transition metals. The size of the exohedral ligands attached to the metal atom also plays a role in deciding the stability of the hypercloso structure. The interaction between the borane and the metal fragments in the hypercloso geometry is analyzed using the fragment molecular orbital approach. The interconversion of the closo and hypercloso structures by the addition and removal of the electrons is also discussed in terms of the correlation diagrams.