CY 211 - Basic Inorganic Chemistry - Boron - PDF

Boron – non metal – electron deficient Boron exists in several allotropes. The three solid phases for which crystal structures are available contain the icosahedral (20-faced) B12 unit as a building block (Fig. 13.1). This icosahedral unit is a recurring motif in boron chemistry. 12 -vertices, 20-t...

Boron – non metal – electron deficient Boron exists in several allotropes. The three solid phases for which crystal structures are available contain the icosahedral (20-faced) B12 unit as a building block (Fig. 13.1). This icosahedral unit is a recurring motif in boron chemistry. 12 -vertices, 20-triangular faces, 30- edges.  Boron – non metal – electron deficient Boron is unique among the elements since it contains fewer electrons (1s2, 2s2 2p1) than atomic orbitals available for bonding It forms covalent bond (no metallic bond) due to (1) small size (2) higher ionization energy This leads to – structural complexity and allotropic modification Boron exists in several allotropes. The three solid phases for which crystal structures are available contain the icosahedral (20-faced) B12 unit as a building block (Fig. 13.1). This icosahedral unit is a recurring motif in boron chemistry. 12 -vertices, 20- triangular faces, 30- edges. Amorphous B is a brown powder but the hard and refractory crystalline B forms shiny black crystals Crystalline “B” Amorphous powder “B”  Boron Hydrides - Diborane Although the existence of BH3 has been established in the gas phase, it has propensity to dimerize to form B2H6 [diborane(6)]. Two common features of boron hydrides are that the B atoms are usually attached to more than three atoms and that bridging H atoms are often present. The bonding in these compounds is not readily described in terms of VB theory. The structure of B2H6 (D2h symmetry) is shown in Fig. 5.31. Features of particular interest are that: ▪ despite having only one valence electron, each bridging H atom is attached to two B atoms; ▪ despite having only three valence electrons, each B atom is attached to four H atoms; ▪ the BH bond distances are not all the same and suggest two types of BH bonding interaction.  Structure of diborane Often, B2H6 is described as being electron deficient; it is a dimer of BH3 and possesses 12 valence electrons. The formation of the BHB bridges can be envisaged as in diagram 5.10. Whereas each terminal BH interaction is taken to be a localized 2c-2e bond, each bridging unit is considered as a 3c-2e bonding interaction. Each half of the 3c-2e interaction is expected to be weaker than a terminal 2c-2e bond and this is consistent with the observed bond distances. Bonding pictures for B2H6 which assume either sp3 or sp2 hybridized B centres are frequently adopted, but this approach is not entirely satisfactory.  MO of diborane The simplified molecular orbital treatment provides insight into the distribution of electron density in B2H6. Using the ligand group orbital approach, we can consider the interactions between the pair of bridging H atoms and the residual B2H4 fragment as shown in Figures below. (1s2, 2s2 2p1) (a) The structure of B2H6 can be broken down into H2B---BH2 and H---H fragments. (b) The ligand group orbitals (LGOs) for the H---H fragment. (c) The six lowest energy LGOs for the B2H4 unit; the nodal plane in the b2u orbital is shown.  MO of diborane An important conclusion of the MO model is that the boron–hydrogen bridge character is delocalized over all four atoms of the bridging unit in B2H6. Since there are two such bonding MOs containing four electrons, this result is consistent with the 3c-2e B-H-B model that we described earlier. (1s2, 2s2 2p1) The terminal B–H bonds and the bridging B–H–B bonds each contain two electrons  The Group 18 Elements Helium, neon, argon, krypton, Xenon, radon, and Oganesson are all monatomic gases. Ground-state valence electron configurations, ns2np6. Helium makes up 23 per cent by mass of the Universe and the Sun, and is the second most abundant element after hydrogen; it is rare in the atmosphere because its atoms travel fast enough to escape from the Earth and too light to be retained by gravity of earth All the other noble gases occur in the atmosphere.  Source of Helium Helium was first detected spectroscopically in the Sun’s atmosphere. Nuclear fusion reactions taking place in the Sun start at temperatures above 107 K, and the following reactions are believed to be the main source of the Sun’s energy (β+ = positron, νe = neutrino):  Clathrates of quinol Obtained by crystalizing quinol from aqueous or other solvent in presence of noble gas at a pressure of 10-40 atm. The quinol crystallizes in the less common β -form, the lattice of which is held together by hydrogen bonds in such a way as to produce cavities in the ratio 1 cavity: 3 molecules of quinol. Molecules of gas (G) are physically trapped in these cavities, there being only weak van der Waals interactions between “guest” and “host” molecules. The clathrates are therefore nonstoichiometric but have an “ideal” or “limiting” composition of [G{C6H4(OH)2}3]. Once formed they have considerable stability but the gas is released on dissolution or melting. Noble gas hydrates Noble gas hydrates are formed similarly when water is frozen under a high pressure of gas. They have the ideal composition, [G8(H2O)46], and again are formed by Ar, Kr and Xe but not by He or Ne. A comparable phenomenon occurs when synthetic zeolites (molecular sieves) are cooled under a high pressure of gas, and Ar and Kr have been encapsulated in this way. Samples containing up to 20% by weight of Ar have been obtained. Xenon fluorides  Reactions of xenon fluorides Both XeF4 and XeF6 act as F− acceptors. The ability of XeF4 to accept F− to give [XeF5]− has been observed in reactions with CsF and [Me4N]F. The [XeF5]− ion (18.12) is one of only two pentagonal planar species known, the other being the isoelectronic [IF5]2−  The structures of the xenon halides - the VSEPR model Structures of xenon fluorides cannot be explained by the conventional valence bond theory. The conventional valence bond theory is based on elements acquiring the octet. The noble gases already have the octet configuration and the compounds they form are hypervalent compounds in which the valence electron count is an even number above 8. Xe 5s 5p 5d GS     ES      sp3d  The structures of the xenon halides - the VSEPR model In the vapour state, the vibrational spectrum of XeF6 indicates an octahedron distorted by a stereochemically active lone pair in the centre of one face XeF6 is seven electron-pair molecules, and may theoretically have an octahedral structure (where the lone pair is in a spherically inactive s-orbital) or a distorted octahedral structure (where the lone pair is active). In fact, XeF6 is fluxional in the gas phase, interchange between structures where the lone pair points through the centre of an F3 triangle (one face) of a distorted octahedral XeF6 molecule. Assignment questions from SKD: 1. Write notes on nuclear fission and nuclear fusion. (2.5) 2. Solubility product and common ion effect are interrelated. Explain this. (2.5) Assignment questions from KM: 3. Structures of xenon fluorides cannot be explained by the conventional valence bond theory. Why? (2.5) 4. Draw the MO diagram of XeF2 and explain 3c-2e bond in it. (2.5) Molecular-orbital representation of the 3-centre F-Xe-F bond. (a) The possible combinations of colinear px, atomic orbitals. and (b) the energies of the resulting MOs (schematic).