Lecture_3_MO

Lecture_3_MO - Chem 310 Lecture Module 3 Molecular Orbital...

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Chem 310 Lecture Module 3 Molecular Orbital Theory
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Molecular Orbital Theory Bonding in Coordination Compounds Valence bond (VB) theory, Linus Pauling (1930s) a)Valence bond (VB) theory, Linus Pauling (1930s) b) Crystal field (CF) theory (Bethe, 1929; van Vleck, 1935) Î ligand field (LF) theory (1950s) c) Molecular orbital theory Î present Indirect evidence against ionic M-L bonds, i.e. for covalent bonding: nephelauxetic effect. Electron-electron repulsion in complexes is less than in free ions, which is true for the ligands as well as the M ions. Reason for the decrease in electron repulsion lies in the formation of covalent M-L bonds, note the effective distance between the electrons increases upon bond formation leading to rge molecular orbitals formed by M and L orbitals large molecular orbitals formed by M and L orbitals. The total nephelauxetic effect in ML n is proportional to the product h L . k M (see Table 11.10) and ligands most effective in delocalizing metal electrons display largest h . Ligand h metal k F - 0.8 Mn 2+ 0.07 H O 1.0 V 2+ 0.1 2 urea 1.2 Ni 2+ 0.12
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Other evidence for covalent M-L bonds: electron paramagnetic resonance spectroscopy (EPR). EPR spectra show peaks depending on the spins of unpaired electrons . An unpaired electron with o interaction with other unpaired electrons or magnetic nuclei will give a single peak Most EPR no interaction with other unpaired electrons or magnetic nuclei will give a single peak. Most EPR spectra of complexes exhibit hyperfine splitting pattern, which proves interaction of the M centered unpaired electron with magnetic nuclei of the ligands. EPR spectrum of K 2 [IrCl 6 ] Molecular orbitals for an octahedral complex Bonding is determined by mixing orbitals on the metal with ligand orbitals (for sigma bonding these are usually s-type orbitals, or s-p hybrids of some sort). nergy and symmetry dictate what mixing will work. E.g. [Co(NH 3 ) 6 ] 3+ . M valence orbitals: 4 s , 4 p , 3 d are of appropriate energy to bond with ligands Î Available L orbitals: one lone pair per NH 3 ( sp 3 hybrid) -> σ M-L bonding. OW fi d li bi ti f M d L bit l ith th t Energy and symmetry dictate what mixing will work. • NOW: find linear combinations of M and L orbitals with the same symmetry. •Must determine linear combinations of the L orbitals ( ligand group orbitals LGO ) which can overlap with the M orbitals along x , y , and z , i.e. the octahedral bonding axes.
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How do we construct symmetry representations for LGO’s? Î group theory E8 C 6C’ 6C 3C i 6S 8S 3 σ 6 σ O 3 2 4 2 4 6 h d h 6 002 20 0 04 2 2 edges C’ es) C 2 12 edges 6 C 2 axes) educes to A ingly degenerate) T riply degenerate) E oubly degenerate) σ d athematical representations of LGO’s are from normalizing combinations of orbitals Î Reduces to A 1g (singly degenerate) T 1u (triply degenerate) E g (doubly degenerate) Î This forms the symmetry adapted LGO set Î The total number of LGO orbitals must = the number you start with Mathematical representations of LGO’s are from normalizing combinations of orbitals (see page 156 of text) I.e.: e g set
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This note was uploaded on 02/28/2011 for the course CHEM 310 taught by Professor Nazar during the Fall '09 term at Waterloo.

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Lecture_3_MO - Chem 310 Lecture Module 3 Molecular Orbital...

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