Chemical Bond Reason for bonding between atoms = a lower total energy of bound atoms than the sum of energies of sepatated atoms Mechanism of bond formation = sharing, transfer, and redistribution of valence electrons • Model of localized electron pairs (Lewis, VB, VSEPR, hybridization) • Model of delocalized electrons (MO) i Chemical Bond Ionic Covalent Metallic Types of Chemical Bond Covalent = sharing of electrons (e pairs, 1e H2+) by several atoms (2, 3, 4....), three-center-two-electron bonds Metallic = sharing of electrons among many atoms, band theory Ionic = transfer of electrons, formation of ions, Coulombic attractive forces between oppositely charged ions Van der Waals = Coulombic attractive forces between temporary charges (dipoles) Topological = mechanical joining of molecules (rotaxenes, catenanes, carcerands) 3 Pauling's Electronegativities .o - (1) (2) O) (4) <5) (6) (7) <8) (9) (10) <11 >(12)(13)( 14)( 15)(1S)(17) n * a mX\v itv Key: □ 0.7-1.4 □ 1.5-1.9 □ 2.0-2.9 □ 3.0-4.0 Van Arkel Triangle Smooth transition between ionic and covalent extremes Ax = (Xa - Xb) Bond lonicity i = 1 - exp [-0.21(Xa-Xb)2] Ionic and Covalent Bond LiCI > NaCI > KCl > RbCI > CsCI 4 Covalency incr. Nal > NaBr > NaCI > NaF 4 Covalency incr. AIN > MgO > NaF lonicity incr. Ionic and Covalent Bond Topological Bond rotaxenes, catenanes, carcerands Ionic Bond Na(s) + y2 Cl2(g) NaCI(s) AH°f = -410.9 kJ moh1 exothermic reaction heat 11 Lattice Energy, L Lattice Energy = energy released upon formation of one mole of solid from ions in the gas phase Coulombic attractive force b/w 2 ions Z = ion charges r = distance Lattice Energy L [kJ moh1] Coulombic attractive and repulsive forces in 1 mole of ions M = Madelung constant Accounts for lattice geometry (NaCI, CsCI, CaF2, ZnS,....) 12 L [kJ mol1] F Cl Br I Li 1037 862 785 729 Na 918 788 719 670 K 817 718 656 615 Rb 784 694 634 596 Cs 729 672 603 568 Lattice Energy and Physical Properties Born-Haber Cycle Na+ (g) + CI (g; E(Na) Na (g) + Cl (g) J Vi D(CI-CI) Na (g) + Vi Cl2 (g) } AHsubl Na (s) + Vi Cl2 (g) AH form NaCI (s) Born-Haber Cycle Na(s) -> Na(g) AHsub, = 107.3 kJ moh1 y2 Cl2(g) -> Cl(g) 1/2 D(CI-CI) = 122 kJ moh1 Na(g) Na+(g) + e~ IE(Na) = 496 kJ moh1 Cl(g) + e- -> Ch(g) EA(CI) = -349 kJ moh1 Na+(g) + Ch(g) -> NaCI(s) L(NaCI) = -778 kJ moh1 Na (s) + 1/2 Cl2 (g) -> NaCI(s) AHform = -401.7 kJ moh1 16 Covalent Bond in H2 Bonding energy H-H j Covalent Bond I Atoms share electron pairs to attain electronic octet in their valence shell Gilbert N. Lewis (1875-1946) ČULÚ-* '0 Gilbert N. Lewis 1902 Lewis Structures Lewis Structures - Octets Lewis Resonance Structures Positions of nuclei are unchanged, move electron pairs Bonding situation is described by superposition of all resonance structures Lewis Resonance Structures o— o Se=0 O" 1 0= o Sc-O' O" o O" Se=0 O o— o Se-O O 0 Se- -0 0 t 0" Se =0 0" o o o Bond order = 1.5 Charge on O = -0,5 Formal Charge Oxidation Number = all e moved to more electronegative atom Formal oxidation number for balancing redox equations Not a real charge on an atomu Formal Charge = difference between number of valence electrons on a free atom and valence electrons assigned to an atom in a molecule: free e pairs count full, bonding pairs halved between atoms Formal Charge Atoms strive to attain minimal formal charge, zero at the best Negative formal charge is placed on the most electronegative atom Sum of formal charges in a molecule (ion) equals total charge on a given molecule 27 Formal Charge Formal charges too big THE BEST FORMULA Negative charge resides on a less electronegative atom 29 Molecules with Unpaired Electrons Dimerisation N02- 2 N02 (g) * N204 (g) Kc = 210 o Paramagnetic molecule = Unpaired electrons = Lewis structures insufficiently describe real situation ->• delocalized e VSEPR Model VSEPR = valence shell electron repulsion Empirical set of rules to predict shapes of coordination sphere of atoms and thus the molecular shape (also ions and molecular fragments) for main group elements and transition metals with electron configuration d° or d10. 31 VSEPR Model Molecule = central atom + ligands + free electron pairs Ligands = atoms or groups Ligands have usually higher electronegativity than the central atom (except H or metals) Valence electrons are arranged in pairs: • Bonding electron pairs • Free electron pairs (nonbonding) 32 Central Atom vs. Ligand VSEPR Model Basic shape of coordination sphere of an atom is given by the number of occupied domains = number of bonds (disregarding multiplicity) + number of free electron pairs VSEPR Model Each electron pair occupies part of space around the central atom and excludes (repels) other electrons = Pauli exclusion principle Electron pairs arrange themselves around the central atom so that they are as far as possible from each other to minimize repulsion Free electron pairs occupy larger space around the central atom than bonding electron pairs FREE > BONDING 35 Tetrahedral Molecule of Methane CH4 Place 4 points on a sphere, so that their distances are maximum tetrahedron 36 VSEPR Model Free electron pairs and bonding electron pairs arrange around the central atom to minimize total energy by minimizing repulsion : Central atom + 2 ligands linear Central atom + 3 ligands equilateral triangle Central atom + 4 ligands tetrahedron Central atom + 5 ligands trigonal bipyramidal or square pyramidal Central atom + 6 ligands octahedral Central atom + 7 ligands pentagonal bipyramidal 37 VSEPR 6 AX6 AEX5 AE2X4 AY acy ac v 4* V b i5 AC Y MÄ5 Mca4 At2A3 an/ A tv A T~~ \/ Ab3X2 AX4 A EX3 AE2X2 a v a cv A A3 AbX2 AX2 38 VSEPR Model Final molecular shape = positions of nuclei (disregard the free electron pairs) The volume of space around the central atom occupied by electron pairs decreases: triple > double > single bond Repulsion among electron pairs decreases: free - free > free - bonding > bonding - bonding 39 I AX3: Bond Angle = 120° AEX2: Bond Angle < 120° TRIGONAL PLANAF Class Shape AX, Trigonal planar Examples: SOy BF3, NO^CQg- AXjE Beni(V shaped) Examples: SQ^ PbCI2, SnBr£ Tetrahedral Trigonal pyramidal Bent 44 Trigonal Bipyramid TBP has 2 different types of vertices = 2 chemically different types of substituents or positions ________ 2 axial 3 equatorial Free electron pairs and multiple bonds occupy always equatorial positions Free electron pairs and multiple bonds occupy always equatorial positions ^^^^^^ Trigonal Bipyramidal Molecular shape is given by positions of nuclei ;aw CIF3 ? T-shape 1 Trigonal Bipyramidal (TBP) and Square Pyramidal (SP) Octahedron AX6 Bond angles in octahedron = 90° 49