Chemical Bond Theories Atomic Orbitals Molecules ? Valentim Nunes, Engineering Unit of IPT February, 2018

TOM MOLECULAR ORBITAL THEORY — Robert Mulliken) Valence electrons are delocalized Valence electrons occupy molecular orbitals that extends to all the molecule

TLV VALENCE BOND THEORY Linus Pauling Valence electrons are localized between the atoms (or are isolated pairs). Semi filled atomic orbitals overlap to form the chemical bond.

Valence Bond Theory The Valence Shell Electron Pair Repulsion Model, based on the Lewis structures do not explain the chemical bonds! Does not explain why the F-F chemical bond is weaker than the H-H bond, although both chemical bonds result from the sharing of one electron pair. Accordingly to the TLV, the formation of the H-H covalent bond results from the spatial overlap of two atomic orbital 1s of both atoms.

Valence Bond Theory Sharing of two electrons between two atoms Dissociation energy of bonds Bond lenght H2 F2 436,4 kJ/mol 150,6 kJ/mol 74 pm 142 pm Valence Bond Theory — the electrons in a molecule occupy atomic orbitals of individual atoms. Overlap 2 1s 2 2p Sharing of two electrons between two atoms

Formation of a chemical bond  (Sigma) Two s orbitals overlap

Formation of a  chemical bond Two s orbitals overlap Two p orbitals overlap

Valence Bond Theory Chemical bonds in BF3 Triangular plane angle = 120o

Chemical bond in BF3 Hybridization of atomic orbitals How to explain the three bonds with an angle with 120o using one orbital s and p orbitals that forms angles of 90o? Pauling introduced the concept of: Hybridization of atomic orbitals The hybridization consists in the mixture of atomic orbitals producing a new set of orbitals — Hybrid orbitals

Hybridization — coalescence of two or more atomic orbitals to produce a new set of hybrid orbitals. Coalescence of, at least, 2 atomic orbitals non-equivalent atomic orbitals (ex., s and p). The hybrid orbitals have a different shape of the original atomic orbitals. The number of hybrid orbitals is equal to the number of atomic orbitals that participate in the hybridization process. The chemical bonds are formed by: Overlap of hybrid orbitals with atomic orbitals. Overlap of hybrid orbitals with other hybrid orbitals.

Chemical bond in BF3 redistribution of electrons hybridization 2p 2s Non occupied orbital p Three hybrid orbitals sp2

BF3 The three hybrid orbitals results from the mixture of one s orbital and two p orbitals  3 sp2 Then we have 3 hybrid orbitals semi filled that are used to form the B-F sigma chemical bonds.

BF3 One orbital of each F overlaps with the sp2 hybrid orbital to form para forma the sigma B-F chemical bond.

Chemical bond in CH4 How to justify 4 C—H sigma chemical bonds with an angle of 109o? We must use 4 atomic orbitals — s, px, py, e pz — to form 4 hybrid orbitals.

Chemical bonds in CH4

Hybridization of Orbitals Count the number of isolated pairs and the number of atoms bonded to the central atom. Isolated pairs + Atoms bonded Hybridization Examples 2 3 4 5 6 sp sp2 sp3 sp3d sp3d2 BeCl2 BF3 CH4, NH3, H2O PCl5 SF6

Bonds in Glicine

Bonds in Glicine

bonds in Glicine

Bonds in Glicine

Bonds in Glicine

Multiple Bonds Consider the ethene, C2H4

sp2 hybridization of a carbon atom Fundamental state Electron promotion sp2 hybridization state

The 2pz orbital is perpendicular to the hybrid orbitals plane.

Bonds in C2H4 Pi bond (p) – electronic density above and below the nuclei of atoms. Sigma bond (s) – electric density between the 2 atoms.

π bond in C2H4 The p orbital non utilized in each C contains an electron and this p orbital overlaps with the p orbital of the neighbor atom to form the π bond.

s and π bonds in C2H4

s and π bonds in CH2O

s and π bonds in C2H2

sigma (s) and pi (p) C H O Simple bond 1 sigma bond Double bond 1 sigma bond + 1 pi bond Triple bond 1 sigma bond + 2 pi bonds s ligações = 6 + 1 = 7 p ligações = 1

Consequences of multiple bonds Rotation is restricted along one C=C chemical bond