Boranes Stream
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Magadh University, Bodh-Gaya Dr. Partha Pratim Das Assistant Professor P.G. Department of Chemistry Magadh University, Bodh-Gaya E-Content Title: Boranes Stream: M. Sc. (2nd Semester) Email id: [email protected] Boranes: Boranes or Boron hydrides are the class of compounds having the general formula BxHy. Boron does not react with hydrogen directly to form any of boron hydrides or boranes, but many of these compounds are easily synthesized under special conditions. Due to high affinity of boron towards oxygen, they readily get oxidized by air. Hence, these compounds do not occur in nature. This whole class is derived from the borane (BH3) only, which exists as a transient intermediate and dimerizes to form diborane (B2H6) immediately. All higher boranes consist of boron clusters, which are polyhedral in nature. Besides neutral boranes, a large number 2− of anionic boron hydrides also exist. The general formula for single-cluster boron hydrides are BnHn , BnHn+4, BnHn+6, BnHn+8 and BnHn+10 (n = number of boron atoms) for closo-, nido-, arachno-, hypho- and klado- type, respectively. There are also series of substituted neutral hypercloso-boranes, which have theoretical formula BnHn. Naming of neutral boranes is done with a Greek prefix showing number of boron atoms and number of hydrogen atoms in brackets, while, for naming of anions, hydrogen count is dictated first followed by the boron count and the overall charge in the bracket in the last, in addition prefixes like closo- nido- etc., which are related to geometry and number of framework electrons of a particular cluster, can also be added. 2− Formula B5H9 B4H10 B6H6 Name (IUPAC) Pentaborane(9) Tetraborane(10) hexahydridohexaborate(2−) Type Nido- Arachno- Closo- VBT approach for 3-centre-2-electron bond in borane: Boron hydrides are electron-deficient species. Simplest boron hydride with 3-center-2-electron bond is diborane (B2H6), bacially dimeric unit of BH3 molecules. One s and three p orbitals mix and generate four sp3 hybrid orbitals, three half-filled and one empty. How come dimerization occurs then, as it seems three hybrid orbitals would be taken up by three H atoms? Each boron atom in diborane can use two of its sp3 half-filled hybrid orbitals to bind two hydrogen atoms (terminal-H). Remaining two hybrid orbitals (one half-filled and one empty) are then used to bind bridging-H atoms as depicted pictorially. Dotted lobes represent empty orbitals. Valence electron of bridging hydrogen and one electron from one half filled sp3 hydrid orbital of one boron atom gets delocalized over all three atoms (B–H–B), producing banana shaped 3-centre-2-electron bond. This depiction of banana bond is contradictory to VSEPR theory though, as it deals with bounded valence electrons into regions of space that are localized between two participating nuclei only. In addition, the geometry is also unexplainable as two bonds around each bridging hydrogen should be linear i.e. the B–H–B bond angle should be ~ 180° with straight-line coordination around, according to VSEPR theory. Molecular Orbital approach for 3-Centre-2-Electron Bond in borane: As we want to understand the nature of bonding involved in B–H–B bridges, our focus will be on just this part of the molecule only. Four equivalent sp3 hybrid orbitals of each Boron atoms and two s orbitals of hydrogen atoms construct the symmetry adapted linear combinations (SALCs) of atomic orbitals. Irreducible components for H atom, ΓH = a1g + b3u, and for B atom, ΓB = a1g + b1g + b2u + b3u. Single-electron wave functions and corresponding molecular orbital shapes for B–H–B portion of diborane are depicted above, focussing mainly only on the largest lobes of the four sp3 hybrid orbitals. Energies two nonbonding molecular orbitals unequal as smaller lobes on the opposite side also have a good possibility to * overlap. Bonding and anti-bonding MOs, a1g and a1g , respectively, are formed by mixing of two SALCs of * a1g symmetry from both H and B atom each. Similarly, bonding and anti-bonding MOs, b3u and b3u , respectively, are formed by mixing of two SALCs of b3u symmetry from both H and B atom each. Shapes of MOs a1g and b3u symmetry clarifies that electron density present in them is delocalized over all three nuclei involved in B–H–B unit of diborane. Some extent of direct B–B bonding, due to small-size of H-atom, is also present in diborane, which helps to stabilize the molecule geometry. Differrent types of bonds present in higher boranes: Overall bonding of higher boranes are explained by counting “framework electrons” i.e. electrons involved in skeletal structure of Bn cluster, where terminal B– H bonds are discarded and B–H–B bridges are considered as part of the Bn framework. Each B atom uses one electron in traditional 2-center-2-electron terminal B–H bond, whereas, remaining two electrons are used in Bn framework. Concept of localized 3-centre-2-electron and 2-centre-2-electron orbital approach is used to explain main features of boranes, though, electrons involved in the framework construction of Bn cluster are very much delocalized. Structures of higher boranes have the following different kinds of bonds: 1. Terminal B–H bond: It is a normal 2-center-2-electron covalent bond shown as B–H. 2. Direct B–B bond: It is a 2-center-2-electron bond connecting two boron atoms, represented as B–B. 3. Bridging or open B–H–B bond: It is a 3-center-2-electron bond connecting two boron and one hydrogen atoms, represented as B–H–B. 4. Closed B–B–B bond: It is a 3-center-2-electron bond connecting three boron atoms present at the corners of an equilateral triangle, depicted as B–B–B. 5. Open bridging B–B–B bond: It is a 3-center-2-electron bond connecting three boron atoms to form a B– H–B like bridge, found in some diborane, represented as B–B–B. Geometry prediction for higher boranes using lipscomb’s model of STYX Numbers: American chemist, W. N. Lipscomb developed a method to determine possible key combinations of bonding profile for a specific boron hydride. His topological model involving STYX numbers and rules was the initial approach to understand the structures of boranes and related species. Each boron atom in neutral boranes or hydroborate anions has at least one H attached by a traditional 2-center-2-electron sigma bond. Hence, one B–H bond is present per boron atom. Further, he proposed that every other bond present in the cluster must belong to any of the following categories: (i) 3-centre-2-electrons B–H–B bond, indicated as “S”. (ii) Closed, open or a mixture of both 3-centre 2-electrons B–B–B bond, indicated as “T”. (iii) 2-center-2-electrons B–B bond, indicated as “Y”. (iv) 2-center-2-electrons B–H terminal bond (Number of BH2 groups), indicated as “X”. Prediction of structures of borane clusters by Lipscomb’s model involves following three major components: (1) Relationship between STYX code and valence electrons: Correlation between borane’s formula and the number and types of bonds in the cluster is given by simple equations of balance, which relate all kinds of sigma bonds to the number of valence electrons available. (a) Three centers orbital balance: Electron deficiency in a borane cluster can be removed only if one 3-center-2-electron bond is created by each boron atom. Hence, sum of the number of 3-centre 2- electron B–H–B bonds and the number of 3-centre 2-electron B–B–B bonds must be equal the number of B–H units (n) i.e. [ = S + T]. (b) Hydrogen balance: Assuming one terminal hydrogen is attached to each boron atom, number of hydrogen atoms leftover (m), must be distributed among bridges and additional B–H terminal bonds i.e. [ = S + X]. (c) Electron balance: If each BH unit contributes one pair of electrons to the skeleton and each of the extra hydrogens gives one electron; number of electron pairs contributed by additional hydrogens must be calculated by dividing their number by two. All of these electron pairs must be participating in bonding; and therefore, the total number of bond pairs can be given as: + ( /2) = S + T + Y + X; Hence, Y = ½ (S-X) (2) Calculation of various STYX possibilities: Upon knowing all correlations between various kinds of bonds with framework electron pairs in boron hydrides, one need to follow the following steps to determine various STYX possibilities. (a) Note down the general formula of given borane cluster in form like BnHn+m to calculate the values of n and m. (b) Determine the number of B–H–B bridges, represented by S. Value of S must lie within the range: [m/2 to m]. Therefore, value of S must satisfy the condition; m/2 ≤ S ≥ . Validation of the higher limit comes from the fact that, m represents total additional hydrogen atoms, out of which some are present in B–H–B bond and some are present in B–H terminal bond. Hence, if all the additional hydrogen atoms are present in B–H–B bond, the value of “S” at its maximum can just be equal to m. Validation of the lower limit can be derived as; + ( /2) = S + T + Y + X + ( /2) = ( − X) + T + Y + X as = S + X = ( /2) + T + Y S + T = ( /2) + T + Y as = S + T S = ( /2) + Y So, S is always equal to or greater than (m/2).