CHM 501 Lecture

First row diatomics: Li2 to Ne2

Note that 1* and 2 are close in energy and of the same symmetry () thus they can overlap (called a configuration interaction); overlap causes an energy change - an avoided crossing that changes the order of the orbitals:
X2
configuration
Bond Order
predicted spin
observed spin
Li2 12
1
0
0
Be2 121*2
0
0
-
B2 121*222
1
2
2
C2 121*24
2
0
0
N2 121*2224
3
0
0
O2 121*2224*2
2
2
2
F2 121*2224*4
1
0
0
Ne2 121*2224*42*2
0
0
-

Neither B2 nor C2 has bond, only bonds so the bond energies are quite low

Better:

Heteronuclear diatomics:

CO as an example (very important molecule in inorganic chemistry; it has the strongest homonuclear bond energy and is a very good Lewis base towards low valent metals)

basis set is still 2s, 2p but now the orbital energies are different

Hybridization to form sp hybrids gives required lone pairs and then form molecular orbitals

21nb242nb2

BO = 3

valence orbital is a lone pair on C so the basic end is C