1 Bonding and Molecular Structure

2 Valence Bond Theory In covalent bonding, orbitals overlap Most primitive overlap between 2 s- orbitals –sigma (σ) bond Electron density greatest along bond axis

3 VB Theory Tenants Orbitals overlap to form a bond between 2 atoms 2 electrons, of opposite spin, can be accommodated in the overlapping orbitals Higher probability of electrons being found in space influenced by both nuclei

4 What about more complex VB’ing? HF still forms sigma bond, but “skewed” –Why? –Which valence electrons of F are involved in sigma bond? (Hint: what’s F’s electron configuraiton?) s-orbital bonds with p-orbital of F to form sigma bond

5 What about VB’ing in orbitals above 1s? Orbital hybridization –Mixing of s-, p-, and d-orbitals within atom to form molecule Two rules: 1) # hybrid orbitals = # atomic orbitals mixed 2) hybrid orbitals more directed from central atom toward terminal atoms than unhybridized atomic orbitals  better orbital overlap & stronger bond between central & terminal atoms Mixed hybridized orbitals yield electron- pair geometry (VSEPR)

6 Hybridized Orbitals 2 electron pairs –s-orbital and 1 p-orbital mix  sp (linear) 3 electron pairs –s-orbital and 2 p-orbitals mix  sp 2 (trigonal planar) 4 electron pairs –s-orbital and 3 p-orbitals mix  sp 3 (tetrahedral) 5 electron pairs –s-orbital, 3 p-orbitals, and 1 d-orbital mix  sp 3 d (trigonal bipyramidal) 6 electron pairs –s-orbital, 3 p-orbitals, and 2 d-orbitals mix  sp 3 d 2 (octahedral)

7 Hybridized Orbitals: sp 3

8 What about electron geometries? For instance, NH 3, H 2 O? Lone pairs fill hybridized orbitals –So, NH 3 = sp 3 –And, H 2 O = sp 3

9 Practice Give the hybridization of and energy diagram for –BeCl 2 –BBr 3

10 Double bonds 1 unhybridized p-orbital (on each atom) allows for overlap Electron densities above and below sigma bonds (but not on axis like sigma bonds) –pi-bond Can only exist if all atoms lie on same plane –Ethylene

11 Ethene

12 Triple bonds 2 unhybridized p-orbitals (on each atom) allow for overlap 2 pi-bonds formed –Perpendicular to one another

13 Ethyne

14 Recapitulate Double bond consists of sigma- and pi-bonds Triple bond consists of sigma- and 2 pi-bonds Pi-bond if unhybridized p-orbitals remain on bonding atoms Multiple bonds entail either sp or sp 2 hybridization