BA + -- (Bonding) 2p z Atomic Orbitals of Atoms A,B zz AB 2p Molecular Orbitals A B  *(Antibonding) 2p z Nodal Plane 2p z -2p z AB 2p z (A) + 2p z (B) 2p z (A) - 2p z (B) z Axis is taken to be line through A, B z axis z axis Bonding with 2p Orbitals

Atomic Orbitals of Atoms A,B Molecular Orbitals AB p x -2p x AB p x x  (bonding) AB + - AB (antibonding) ** 2p Nodal Plane 2p x (A) - 2p x (B) 2p x (A) + 2p x (B) z x x z Note: orbital is  to bond (z) axis A,B Nodal Plane Bonding with 2p Orbitals (cont)

Notational Detail Oxtoby uses two different notations for orbitals in the 4th and 5th editions of the class text: 2px* in the 4th edition becomes g2px * in the 5th edition 2px in the 4th edition becomes u2px in the 5th edition 2pz in the 4th edition becomes   g2pz in the 5th edition 2pz* in the 4th edition becomes   u2pz * in the 5th edition

CORRELATION DIAGRAM f or second period diatomic molecules E ** 2p z ** y ** x  x  y  z ** z ** x ** y  z  x  y 2s **  2p 2s **  (6 Valence electrons for each atom) (3 Valence electrons for each atom) B 2 Z ≤ 7 O 2 Z ≥ 8 Molecular OrbitalsMolecular Orbitals Note unpaired electrons Note order of filling  2p z Degenerate  orbitals

Molecular Orbitals of Homonuclear Diatomic Molecules (Å) He 2 Li 2 Be 2 B2B2B2B2 C2C2C2C2 N2N2N2N2 O2O2O2O2 F2F2F2F2 Ne Valence Electron Configuration # Valence Electrons Bond Length Bond Order BondEnergy(kJ/mole) Mole-cule H2H2H2H2 (  2s ) 2 (  2s *) 2 (  2p z ) 2 (  2p ) 4 (  2p *) 4 (  2s ) 2 (  2s *) 2 (  2p z ) 2 (  2p ) 4 (  2p *) 2 (  2s ) 2 (  2s *) 2 (  2p ) 4 (  2p z ) 2 (  2s ) 2 (  2s *) 2 (  2p ) 4 (  2s ) 2 (  2s *) 2 (  2p ) 2 (  2s ) 2 (  2s *) 2 (  2s ) 2 (  1s ) 2 (  1s *) 2 (  1s ) 2 (  2s ) 2 (  2s *) 2 (  2p z ) 2 (  2p ) 4 (  2p *) 4 (  2p z *) 2          

Bonding in Polyatomic Molecules Basically two ways to approach polyatomics. First is to use delocalized M.O.’s where e - are not confined to a single bond (region between 2 atoms) but can wander over 3 or more atoms. We will use this approach later for C bonding. Second is to use hybridization of atomic orbitals and then use these to form localized (usually) bonds.

Localized BeH2 orbitals: + Be sp + sp - Add sp+ + 1s to get localized Be-H bond } Add sp- + 1s to get localized Be-H bond } HH.... – – + +

H 1s + (sp- )H 1s + (sp+ ) BeHHBe and ––

BH3 Fragment: sp 2 hybrid ˚ Boron Nucleus Second sp 2 points along this direction Third sp 2 points along this direction

3 sp2 hybrids 120˚

Overlap with H 1s to give 3 B-H bonds in a plane pointing at 120º with respect to each other: BH3 B Note B has 3 valence electrons 1s22s22p H +H+H 120˚

2s + 2p x + 2p y + 2p z  sp 3 gives 4 hybrid orbitals which point to the corners of a tetrahedron. Angle between is 109º28’ sp 3 [1/4 s, 3/4 p]. Tetrahedral hybrids. 4 H atoms have 4 x 1s  4 valence e -

E 2s 2 2p 1s 2 sp 3 1s 2

Geometry of carbon sp3/H 1s Bonds in methane (CH4): sp3 hybridization on C leads to 4 bonds. CH4 is a good example. C H H H H

Summary of Hybridization Results BeH22 splinear H-Be-H BH33 sp2trig. plane (120º H-B-H angle) CH44 sp3tetrahedral (109º28’ H-C-H angles) ExampleGroups Attached HybridGeometry to Center Atom

2p z 2p y 2p x 1s Localized Bonds and Lone Pair Electrons NH 3 This predicts 90º geometry 3H, 1s (no choice)N: 1s 2 2s 2 2p 3 Nitrogen nucleus H atom 1s orbital

N 107˚ H H H Geometry of NH3 found to be Trigonal Pyramidal

4 N sp3 orbitals combine with 3 1s H orbitals to give 3 sp3, 1s M.O.’s leaving one sp3 hybrid left Of 5 valence e - in N, 2 go into one sp3 orbital, 3 go into other 3 sp3,s. (combined with H (1s)) One of the driving forces for the tetrahedral configuration is that it puts bonding and lone pair electron groups as far away from each other as possible.

N H H H Lone Pair

N Electron pair repulsion effect is largest for small central atoms like B, N, O. As go to larger central atoms (e.g. S or metals) frequently find this effect not so large and start to get things closer to pure p orbital bonds (90º structures) For example, H2S has H-S-H angle of 92º.

O H H Lone pairs stick into and out of paper H2O: 4 sp3 hybrids. Use 2 to make MO bonds with H 1s. Predicts H-O-H bond of 109º28” - Find 105º O 1s 2 2s 2 2p 4 (6 valence e - )2H 1s (2e - )