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Lecture 17 Molecular Orbital Theory 1) Molecular Orbitals of AH x (x = 3, 4, 6)

Lecture 17 Molecular Orbital Theory 1) Molecular Orbitals of AH x (x = 3, 4, 6). MO diagrams can be used on a qualitative basis to understand the shape of molecules of the same stoichiometry (same x) but formed with different central atoms A (most commonly, should belong to the same period)

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Lecture 17 Molecular Orbital Theory 1) Molecular Orbitals of AH x (x = 3, 4, 6)

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  1. Lecture 17 Molecular Orbital Theory 1) Molecular Orbitals of AHx (x = 3, 4, 6) MO diagrams can be used on a qualitative basis to understand the shape of molecules of the same stoichiometry (same x) but formed with different central atoms A (most commonly, should belong to the same period) Examples: removing electrons one by one from the highest occupied molecular orbital (HOMO) and decreasing nuclear charge of A we can get: AH: from MO’s of HF – MO’s of HO●, HN (triplet and singlet), HC, HB (triplet and singlet), etc.; AH2: from MO’s of H2O – MO’s of H2N●, H2C (singlet and triplet), H2B●, BeH2 etc. AH3: from MO’s of H3N – MO’s of H3C●, H3C+, BH3 etc. AH4: from MO’s of CH4 – MO’s of NH4+, NH4+● etc. While doing so, keep appropriate Walsh diagrams handy and take into account possible changes in s-p orbital mixing.

  2. 2) Molecular Orbitals of NH3 (C3v) • NH3 (C3v: E, 2C3, 3sv) The symmetry of 3H’s group orbitals: Gr = 3E+0C3+sv = A1 + E

  3. 3) Molecular Orbitals of CH4 (Td) The symmetry of 4H’s group orbitals: Gr = 4E+1C3+0C2+0S4+2sd = A1 + T2

  4. 4) Molecular Orbitals of closo-B6H62- (Oh)

  5. 5) Molecular Orbitals of closo-B6H62-. “Radial” group orbitals + Note that only one of the six 2pzboron group orbitals, namely a1g, is bonding Six 2s and six 2pz boron group orbitals will mix to form two sets of radial orbitals. One of these twosix-orbital sets will be used to combine with six 1s hydrogen group orbitals to form six bonding and 6 antibonding MO’s (B-H bonds)

  6. 6) Molecular Orbitals of closo-B6H62-. “Tangential” group orbitals • Remaining twelve 2px and 2pyboron orbitals form four sets of triply degenerate “tangential” group orbitals of t1g, t2g, t1u and t2usymmetry. • Only two of these sets ,t2g and t1u, are suitable for B-B p-bonding in closo-B6H62-. They form six p-bonding MO’s (B-B p-bonds).

  7. 7) B-B and B-H bonding MO’s of closo-B6H62- • closo-B6H62- has 7 core bonding orbitals, 6 of them are p- (t1u & t2g) and one is s-MO (a1g). • In boron cages of the formula closo-(BH)x (x = 5, … 12) the optimum number of the core electron pairs is x+1 (all bonding orbitals are filled). That explains enhanced stability of dianionic species closo-(BH)x2-.

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