Clusters and catenation in p-block

Transcription

1 Clusters and catenation in p-block Allotropes of carbon Allotropes of boron Part of one layer of the infinite lattice of α- rhombohedral boron, showing the 12 icosohedral building blocks that are covalently linked to give a rigid infinite lattice The construction of the 84 -unit, the main building block of the infinite β-rhombohedral boron. (a) In the centre of the unit is a 12 -icosohedron, and (b) to each of these twelve, another boron atom is covalently bonded. (c) A 60 -cage is the outer skin of the 84 -unit. (d) The final 84 -unit can be described in terms of covalently bonded sub-units { 12 }{ 12 }{ 60 }. CM 2312 Clusters and Catenation 1

2 Isoelectronic species: Molecules, ions or molecular fragments that possess the same number of valence and core electrons are said to be isoelectronic. owever, this term is often used to refer to species that simply possess the same number of valence electrons. Example: N is isoelectronic with CC The structure of layered, hexagonal boron Nitride, note that the atoms are in register between layers The sphalerite structure of boron nitride -N analogues of hydrocarbons + C N 8 valence electron series CM 2312 Clusters and Catenation 2

3 ONDING IN ORANES 2 6 : The boron atom in 3 possesses a vacant 2p atomic orbital that readily accepts a pair of electrons from a Lewis base (L:). In the absence of a Lewis base, 3 will dimerise by the donation of a pair of - bonding electrons from one molecule into the empty 2p orbital of the other. The two bonding electrons are now shared between three atoms and a -- bridge is formed. :L L = 2e - donor L 3 3 dimerisation Number of atoms in bonding contact in 2 6 = 8 need 8 x 2 = 16e if all bonds are 2e/2 centre Number of valence electrons = (2 x 3) + (6 x 1) = 12 all bonding contacts between pairs of atoms are not 2e/2 centre Polyhedral oranes e.g. closo-[ 6 6 ] 2- [ 6 6 ] 2- is an octahedral (O h ) cluster of 6 boron atoms, each of which is bonded to a terminal hydrogen atom. 2- / A prototype for the consideration of bonding in boron cages CM 2312 Clusters and Catenation 3

4 Unit used for bonding in polyhedral borane clusters x z sp hybridised x 6 SALC a 1g + e g + t 1u = 6 orbitals y p x, p y x 6 t 1g + t 1u + t 2g + t 2u = 12 orbitals Molecular orbital scheme for a 6 cluster in O h point symmetry t 1u Relative energy LUMO OMO e g t 1g t 2u t 2g t 1u - Anti-bonding - onding Thus, a 6 cluster possesses 7 strongly bonding orbitals. Therefore, [ 6 6 ] 2 is stable as a 2 anion. This is a general result. n n (n = 6 to 12) clusters possess (n + 1) bonding orbitals and are stable as [ n n ] 2-. a 1g The seven bonding orbital combinations of [ 6 6 ] 2 CM 2312 Clusters and Catenation 4

5 [ n n ] 2- clusters have a deltahedral boron cage, i.e. the polyhedra possess only triangular faces, and each boron atom is bonded to a terminal atom. CM 2312 Clusters and Catenation 5

6 The cage structure of the boron hydrides (boranes), their carbon-substituted relatives (carboranes), many clusters of the p-block elements (and many d-transition metal clusters) follows from: The relationship of the number of vertices to the number of electron pairs involved in the cage skeleton: oranes Type Formula Skeletal Examples electron pairs Closo [ n n ] 2- n+1 [ 6 6 ] 2-, [ ] 2- Nido [ n n+4 ] n+2 [ 5 9 ], [ 6 10 ] Arachno [ n n+6 ] n+3 [ 4 10 ], [ 5 11 ] Closo Nido Arachno CM 2312 Clusters and Catenation 6

7 Relationship between closo, nido and arachno - structures 1. Closo nido e.g. [ 6 6 ] 2- [ 5 9 ] [ 6 6 ] 2-14 skeletal electrons (7 pairs) - 2 e lose one electron pair [ 6 6 ] 12 skeletal electrons (6 pairs) - lose one vertex [ 5 5 ] 10 skeletal electrons (5 pairs) + 4 added to cage bridging between pairs of boron atoms [ 5 9 ] 14 skeletal electrons (7 pairs) Thus each atom added (beyond the terminal - bonds) brings one electron to the skeletal (-/-) cage. CM 2312 Clusters and Catenation 7

8 2. Closo arachno e.g. [ 6 6 ] 2- [ 4 10 ] [ 6 6 ] 2-14 skeletal electrons (7 pairs) - 2 e lose one electron pair [ 6 6 ] 12 skeletal electrons (6 pairs) - 2 lose two vertices, each with one pair of electrons [ 4 4 ] 8 skeletal electrons (4 pairs) electrons added to skeleton [ 4 10 ] 14 skeletal electrons (7 pairs) (n + 3) arachno structure based on closo minus two vertices 4 of the 6 hydrogen atoms bridge across.... edges and two form terminal - bonds CM 2312 Clusters and Catenation 8

9 A set of guidelines may be followed in order to predict the structure of a borane cluster. 1. ow many [-] units are there? 2. ow many additional atoms are there? 3. ow many valence electrons are available for cluster bonding? 4. What is the parent deltahedron? 5. After each [-] unit has been accommodated at a skeletal vertex, are there any vertices vacant in the parent deltahedron? What class is the cluster? 6. Additional atoms are placed either around the - edges of an open face of the cluster or in extra terminal positions, usually if a boron atom is of particularly low connectivity. 7. In arachno structures vacant vertices are usually adjacent. Cluster symmetry is generally kept as high as possible. Why do bridging hydrogen atoms occupy sites around the open faces of a cluster? The open face of a cluster is generated by removing one, or more, vertices from a deltahedral cluster. Considering MO theory, the loss of a vertex leaves a region of electron density focused around an open face, hence protons will readily interact with the open face. This leads to -- bridges around the open face. onding cluster MO derived from [ 6 6 ] 2-1s AO Example CM 2312 Clusters and Catenation 9

10 Example 2. [ 6 9 ]- Example 3. [ ]2- Example 4. C CM 2312 Clusters and Catenation 10

11 ISOELECTRONIC SPECIES Molecules, ions or molecular fragments that possess the same number of valence and core electrons are said to be isoelectronic. owever, this term is often used to refer to species that simply possess the same number of valence electrons. With respect to the number of valence electrons available the following fragments are isoelectronic with each other: e.g. Carboranes {-}, {C}, {CMe}, {N+} The Isolobal Principle It is possible to convert a borane into a carborane cluster by replacing a { } by a {C} unit because both fragments possess the same frontier orbital properties. The two sets of MOs have the same symmetry characteristics, are of approximately the same energy, and contain the same number of electrons, 3, available for cluster bonding. The { } and {C} fragments are said to be isolobal. The principle can be extended to include a range of atoms and molecular fragments all of which exhibit similar frontier orbitals. Series of isolobal p-block fragments are: (a) (b) (c) { }, {R }, {C}, {CR}, {NR + }; (where R is a one electron donor e.g. alkyl, aryl, halogen atom) Contribute 3 orbitals and 3 electrons for skeletal bonding {R}, {AlR}, {GaR}, {GeR + }, {SR 3+ }; (where R is a one electron donor e.g. alkyl, aryl, halogen atom) Contribute 3 orbitals and 2 electrons for skeletal bonding {}, {Ge}, {Sn}, {N + }, {i + }, {Se 2+ }; (where each bare atom carries an exo-lone pair of electrons. Contribute 3 orbitals and 2 electrons for skeletal bonding CM 2312 Clusters and Catenation 11

12 Zintl Ions A Zintl phase, M n E x, is formed between a very electropositive metal (M) and a less electropositive metal E), e.g. between a Group 1 metal such as Na and a heavy p-block metal such as Tl. Early syntheses involved dissolving the p-block metal in liquid ammonia containing Na. Current syntheses use special chelating ligands. Syntheses The structures of the Group 14 anionic clusters [E 4 ] 2- (tetrahedron, [E 5 ] 2- (trigonal bipyramid), [E 9 ] 2- (tricapped trigonal prism) and [E 9 ] 4- (monocapped square antiprism) CM 2312 Clusters and Catenation 12

13 [Ge 4 ] 4- [Sn 5 ] 2- [Ge 9 ] i i i i Isoelectronic species D 4h [Se 4 ] 2+ Se Se Se Se [i 4 ] 2- CM 2312 Clusters and Catenation 13

14 Reactions of oron Clusters The syntheses of boron clusters involve cage expansion, cage coupling, cage fusion and the introduction of heteroatoms. 1. Closo hydroborate dianions - reactivity similar to aromatic organic molecules [ (CO)] 2- [ + 1,7-(CO) R] ,10-(CO) RCl with AlCl 3 CO in presence of + water soluble salts 2 O X 2 X = Cl, r or I [ x X x ] 2- x = 1 to 12 Electrochemical oxidation in MeCN [ ] 2- ydride [ ] 3- No reaction The reactivity and stability depends on cluster size and the counter-ion. Ag 2 [ 6 6 ] detonates upon heating, but Cs 2 [ 6 6 ] is thermally stable. [ ] 2- and [ ] 2- are hydrolytically stable in aqueous acidic and basic solutions. Substitution is electrophilic Resistant to chemical oxidation All sites are structurally equivalent CM 2312 Clusters and Catenation 14

15 2. Neutral nido boranes: e.g. 5 9 isomerise 1-R (OR) R 5 8 RCl with AlCl 3 1 RO M[ 5 8 ] + 2 Na or K deprotonation 1,2-Cl isomerise 2,3 (or 2,4)- Cl Cl 2, T > 0 o C with Lewis acid nido- 5 9 X 2, X = r or I C 2 2, 500 o C carborane formation closo-c isomerise closo-c X X 5 + closo-c Electrophilic and nucleophilic attack Electrophilic attack at apex followed by conversion to basal isomer Deprotonation at bridge 3. Neutral arachno boranes: e.g (O) O 2-r r r 2 at -15 o C Na or K M[ 4 9 ] + 2 arachno CO or PF 3 C o C 1-(CO) 4 8 or 1-(PF 3 ) Nucleophilic attack Deprotonation at bridge closo-c closo-c closo-c YDROLYSIS: closo < nido < arachno CM 2312 Clusters and Catenation 15