This book provides qualitative molecular orbital and valence-bond descriptions of the electronic structures for electron-rich molecules, with strong emphasis given to the valence-bond approach. Electron-rich molecules form an extremely large class of molecules, and the results of quantum mechanical studies from different laboratories indicate that qualitative valence-bond descriptions for many of these molecules are incomplete in so far as they usually omit "long-bond" Lewis structures from elementary descriptions of bonding. For example, the usual representation for the electronic structure of the ground-state for 03 involves resonance between the (+1 o and Until standard Lewis structures ~ ~ (-I . b:'" ~d· . . . . , recently, any contribution to resonance of the "long-bond" (or spin-paired o .. / .. ,. . has been largely ignored. diradica~ Lewis structure However, it :0 . 0. . e-. . . . . __ " has now been calculated to be a very important structure. For the ground-states of numerous other systems, calculations also indicate that "long-bond" structures are more important than is usually supposed, and therefore they should frequently be included in qualitative valence-bond descriptions of electronic structure. The book describes how this may be done, and some of the resulting consequences for the interpretation of the electronic structure, bond properties and reactivities of various electron-rich molecules. When appropriate, molecular orbital and valence­ bond descriptions of bonding are compared, and relationships that exist between them are derived.

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1 Atomic Orbitals, Electron Spin, Linear Combinations.- 1-1 Atomic Orbitals.- 1-2 Electron.- 1-3 Linear Combinations of Wave Functions.- 2 Pauling "3-Electron Bonds", 4-Electron 3-Centre Bonding and the Need for an "Increased-Valence" Theory.- 2-1 Introduction.- 2-2 Electron Deficient and Electron Excess Bonding Units.- 2-3 Delocalized Molecular Orbital Theory for 4-Electron 3-Centre Bonding Units.- 2-3(a) Symmetrical 4-electron 3-centre bonding: H3?.- 2-3(b) Non-symmetrical 4-electron 3-centre bonding: N2O and F2O2.- 2-4 Standard Valence-Bond Theory for N2O and F2O2.- 2-4(a) The octet rule and the electroneutrality principle.- 2-4(b) Standard valence-bond theory and F2O2.- 2-5 "Long-Bond" Valence-Bond Structures and a Need for an "Increased-Valence" Theory.- 2-5(a) The electroneutrality principle and "long-bond" valence-bond structure.- 2-5(b) The need for "increased-valence" structures.- References.- 3 Wave-Functions and Valence-Bond Structures for 1-Electron Electron Bonds, Electron-Pair Bonds, Pauling "3-Electron Bonds" and "No Bonds".- 3-1 Diatomic Bonding and Antibonding Molecular Orbitals.- 3-2 One-Electron Bonds.- 3-3 Electron-Pair Bonds.- 3-4 Spin Wave-Functions for One-Electron and Two-Electron Systems.- 3-5 An Important Theorem.- 3-6 The Pauling "3-Electron Bond".- 3-7 Slater Determinants and the Pauling "3-Electron Bond".- 3-8 "No Bonds".- 3-9 Valence-Bond Structures and Bond Properties for H2+, H2, He2+ and He2.- 3-10 Inclusion of Overlap Integrals in Normalization Constants for Molecular Orbitals; Non-Bonded Repulsions.- 3-11 Bond Orders.- References.- 4 Valence-Bond Structures for Some Diatomic Molecules.- 4-1 Molecular Orbital Configurations for Homonuclear Diatomic Molecules.- 4-2 Li2, Be2, N2, F2 and O22?.- 4-3 O2, O+2, O?2 and O2?.- 4-4 CN?, CO and NO+.- 4-5 NO and SN.- 4-6 S2, SO and NO?.- 4-7 ClO and FO.- 4-8 ClF2 and SF3.- 4-9 N-H Bond Strengths of NH3, N2H4 and N2H2, and HN2 ? H + N2.- References.- 5 Pauling "3-Electron Bonds" and Hypoligated Transition Metal Complexes.- 5-1 Hypoligated and Hyperligated Transition Metal Complexes.- 5-2 Pauling "3-Electron Bonds" and the Electronic Structure of [Fe(H2O)6]2+.- 5-3 Metal-Ion Spin-State and Metal-Ligand Bond-Lengths.- 5-4 Interconversion Between Hypoligated and Hyperligated Electronic States.- 5-5 Metal-Ligand ?-Bonding and Pauling "3-Electron Bonds".- References.- 6 Pauling "3-Electron Bonds", 5-Electron 3-Centre Bonding and Some Tetra-Atomic Radicals.- 6-1 NO2.- 6-2 CO2? and BF2.- 6-3 Triatomic Radicals with 19 Valence-Shell Electrons: O3?, SO2?, NF2 and C1O2.- 6-4 3-Centre Molecular Orbitals and Pauling "3-Electron Bonds".- 6-5 Some Tetra-Atomic Radicals.- References.- 7 Some Dimers of Triatomic Radicals with 17 and 19 Valence-Shell Electrons.- 7-1 The Long, Weak N-N Bond of N2O4: Lewis Valence-Bond Theory.- 7-2 The Long Weak N-N Bond of N2O4: Molecular Orbital Theory.- 7-3 The Planetary of N2O4, Covalent-Ionic Resonance and Cis O-O Puling "3-Electron Bonds".- 7-4 C2O42? and S2O42? Anions.- 7-5 B2Y4 (Y = F, Cl or Br), N2F4 and P2F4.- 7-6 The Geometries of P2F4 and S2O42?.- 7-7 C-Nitroso Dimers and S4N4.- References.- 8 Some Cu(II) Binuclear Transition-Metal Cluster Complexes.- 8-1 Cu(II) Carboxylate Dimers, Cu2 (RCO2)4,Ln.- 8-1(a) Valence-bond structures.- 8-1(b) The antiferromagnetism of Cu(II) carboxylate dimers.- 8-1(c) Covalent-ionic resonance and the antiferromagnetism of Cu(II) carboxylate dimers.- 8-1(d) Covalent-ionic resonance and approximate 6-center molecular orbitals.- 8-2 Cu(II)-X-Cu(II) Linkages.- References.- 9 Excited States.- 9-1 H2: ? ? ?; C2H4: ? ? ? Transitions.- 9-2 n ? ? Transitions.- 9-3 CH3NO and O3: "n + ?" Transitions.- 9-4 O2: ?x ? ?y and ?x ? ?x* Transitions.- Reference.- 10 Pauling "3-Electron Bonds" and "Increased-Valence" Theory for N2O4.- 10-1 Pauling "3-Electron Bonds" for NO2 and "Increased-Valence" Structures for N2O4.- 10-2 "Increased-Valence" Structures and Molecular Orbital Theory for N2O4.- 10-3 "Increased-Valence" Theory and Configuration Interaction for N2O4.- 10-4 Conclusions.- References.- 11 Pauling "3-Electron Bonds" and "Increased-Valence" Structures.- 11-1 Pauling "3-Electron Bonds" and 4-Electron 3-Centre Bonding.- 11-2 Nitrosyl Halides.- 11-3 CH3NO and HNO.- 11-4 Some Dioxygenyl Compounds.- 11-5 Some S-O and S-S Compounds.- 11-6 O3, SO2, S2O, NO2.- 11-7 Pauling "3-Electron Bonds" and 6-Electron 4-Centre Bonding: N2O2, Cl2O2, S2O2 and S2I42+.- 11-8 NO2, NO2?, ClO2, SO2? and SO3.- 11-9 N2O4, N2O3 and FNO2.- 11-10 Sym NO3 and Asym N2O4.- 11-11 Conclusions.- References.- 12 "Increased-Valence" Structures Constructed from Lewis Structures: Delocalization of Lone-Pair Electrons into Vacant Bonding Orbitals.- 13 "Increased-Valence" Structures for N-Centre Bonding Units.- 13-1 N20 and some Isoelectronic Molecules and ions with 4-Electron 3-Centre Bonding Units.- 13-2 N2O4, C2O42? and (RNO)2, with 6-Electron 4-Centre Bonding Units.- 13-3 Some Comments on 6-Electron 4-Centre Bonding Units.- 13-4 Cyclic 6-Electron 4-Centre Bonding Units.- 13-5 Branching 6-Electron 4-Centre Bonding Units.- 13-6 6-Electron 5-Centre Bonding Units.- 13-7 8-Electron 6-Centre Bonding Units.- 13-8 "Increased-Valence" Structures for Longer N-Centre Bonding Units.- 13-8(a) S4N3+: 10-electron 7-centre bonding.- 13-8(b) S4N4: 12-electron 8-centre bonding.- 13-8(c) (SN)X: Polymerized Pauling "3-electron bonds" and polymerized 6-electron 4-centre bonding units..- 13-9 Paramagnetic "increased-Valence" structures.- References.- 14 Delocalization of Lone-Pair Electrons into Anti-Bonding Orbitals: "Increased-Valence" Structures, Molecular Orbital Theory and Atomic Valencies.- 14-1 Delocalization of Lone-Pair Electrons into Antibonding Orbitals.- 14-2 Approximate Molecular Orbital Theory for 4-Electron 3-Centre Bonding Units.- 14-3 Atomic Valencies for "increased-Valence" Structures.- References.- 15 Slater Determinants and Wave-Functions for "Increased-Valence" Structures.- 15-1 "Increased-Valence" Wave-Functions for 4-Electron 3-Centre and 6-Electron 4-Centre Bonding Units.- 15-2 Spin Degeneracy and Wave-Functions for "Increased-Valence" Structures.- 16 Classical Valence Structures and Quinquevalent Nitrogen Atoms.- References.- 17 Some Tetrahedral Molecules and d?-P? Bonding for Some Sulphur Compounds.- 17-1 d-Orbitals as Polarization and Hybridization Functions.- 17-2 F2SO, F2SS and (CH3)2…