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Molecular Theories. CHEM 210 SPRING 2014. Molecular Theories. Valance Shell Electron Pair Repulsion-VSEPR Ligand Close Packing Theory-LCP Valance Bond Theory-VBT Molecular Orbital Theory-MOT Crystal Field Theory-CFT Ligand Field Theory-LFT. Molecular Theories.
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Molecular Theories CHEM 210 SPRING 2014
Molecular Theories • Valance Shell Electron Pair Repulsion-VSEPR • Ligand Close Packing Theory-LCP • Valance Bond Theory-VBT • Molecular Orbital Theory-MOT • Crystal Field Theory-CFT • Ligand Field Theory-LFT
Molecular Theories • Theories are not exclusive of one another • Theories are not inclusive • They are all complementary • In some cases one theory may not be needed
VSEPR • Widely adopted as the theory used to determine molecular shape in the early 1960’s • Initially introduced in 1940 by NevilSidgwick as the Theory of Valency • Refined by Ronald Gillespie and Ronald Nyholm
VSEPR • Prior to VSEPR molecular shapes were determined by bonded atoms • BF3 and PF3 were assumed to have the same shape • VSEPR takes into account not only the bonded atoms but also any unshared pairs • Shapes are determined using the AXE method to assign a molecular class
LCP • Very similar to VSEPR but used mostly in coordination chemistry • Recently (1997) developed by Ronald Gillespie • In VSEPR the electron pairs repeal each other to shape the molecule • In LCP the strength of the Ligand is the cause of the repulsion/shape
VBT • Bridges the chasm between Atomic Orbitals and Molecular Orbitals • Assumes that electrons are donated from each atom (covalent bonds) • No electrons are unpaired in Oxygen • Assumes localized bonds • Coordinate covalent bonds (ligands)
VBT • The idea of sharing electrons is introduced by Gilbert Lewis in 1916 • 1925-1926 Schrodinger Publishes his Wavemechanics work • 1927 Heitler and London successfully apply the SE to describe the properties of H2 • 1928-1930 Linus Pauling introduces Resonance and Hybridization
MOT • Used to describe the stability of a molecule • Accurately describes the properties of a molecular bond • It does not provide any information about hybridization of atomic orbitals • It does not assign electrons to any given atom (remember… they are clouds)
MOT • Much harder to comprehend than VBT • Fully accepted as a valid Theory in in 1950 • Based on work done by • FriederichHund (Hund’s Rule) • Robert Mulliken (Orbitals) • John Slater (LCAO) • John Lennard-Jones (L-J potential) • Charles Coulson (H2 molecule)
MOT • Molecular orbitals are a “mixture” of atomic orbitals from participating atoms • Since orbitals are described by functions • You get positive and negative mixing • To obtain the “true” identity of the molecular orbital… • Apply the Hartree-Fock Method (SCF approach) • Based on a Linear Combination of Atomic Orbitals
CFT • Developed by Hans Bethe and John Hansbrouck van Vleck in the 1930’s • to explain the effects of ligand on Metal centers (ligand strength) • As a consequence it can also explain… • the color of inorganic compounds • molecular shape preferred by a metal’s complexes • Magnetic properties (high spin vs low spin) • DOES NOT explain bonding • Combined with MOT to create LFT
LFT • It expands MOT to Coordination Chemistry • It is said to be more “accurate” than CFT • It actually describes bonding • Electrostatic interactions between Ligand e-s and d-electrons on the metal.
