5. LLiinneeaarr ccoommbbiinnaattiioonn ooff aattoommiicc oorrbbiittaallss
RRuulleess ffoorr lliinneeaarr ccoommbbiinnaattiioonn
1. Atomic orbitals must be roughly of the same energy.
2. The orbital must overlap one another as much as
possible- atoms must be close enough for effective
overlap.
3. In order to produce bonding and antibonding MOs,
either the symmetry of two atomic orbital must remain
unchanged when rotated about the internuclear line or
both atomic orbitals must change symmetry in identical
manner.
6.
7. Typical molecular energy levels
diagram of an octahedral complex
showing the frontier orbitals in the
tinted box
Singly degenerate s a1g
Triply degenerate p t1u
Doubly degenerate d eg
Triply degenerate d t2g
9. RRuulleess ffoorr tthhee uussee ooff MMOOss
* When two AOs mix, two MOs will be produced
* Each orbital can have a total of two electrons (Pauli
principle)
* Lowest energy orbitals are filled first (Aufbau principle)
* Unpaired electrons have parallel spin (Hund’s rule)
Bond order = ½ (bonding electrons – antibonding
electrons)
12. The accumulation of electron density between the nuclei put the
electron in a position where it interacts strongly with both nuclei.
Nuclei are shielded from each other
The energy of the molecule is lower
31. FFiirrsstt ppeerriioodd ddiiaattoommiicc mmoolleeccuulleess
H s1s2
Energy
H2 H
su*
1s 1s
sg
Bond order: 1
Bond order =
½ (bonding electrons – antibonding electrons)
32. Diatomic molecules: The bonding in He2
He s1s2, s*1s2
Energy
He He 2
su*
1s 1s
sg
Bond order: 0
Molecular Orbital theory is powerful because it allows us to predict whether
molecules should exist or not and it gives us a clear picture of the of the
electronic structure of any hypothetical molecule that we can imagine.
34. Diatomic molecules: Homonuclear Molecules of the Second Period
s1s2, s*1s2, s2s2,
s*2s2
Bond order: 0
Be
Energy
Be Be 2
2su*
2s 2s
2sg
1su*
1s 1s
1sg
45. Heteronuclear Diatomics….
The energy level diagram is not symmetrical.
The bonding MOs are
closer to the atomic
orbitals which are
lower in energy.
The antibonding MOs
are closer to those
higher in energy.
c – extent to which each atomic
orbitals contribute to MO
If cAcB the MO is composed principally of fA
63. Oh σ bonding
4p
4s
3d
Do
x2-y2 z2 xy xz yz NB MOs
Antibonding
MOs
Six donor orbitals
6NH3 each donating
2 e-s
Bonding MOs
Atomic orbitals in metal ion Atomic orbitals Molecular orbitals in ligand ion
Molecular Orbital diagram for [CoIII(NH3)6]3+
64. 4p
4s
3d
x2-y2 z2 xy xz yz NB MOs
Antibonding
MOs
Six donor orbitals
6 F- each donating
2 e-s
Bonding MOs
Atomic orbitals in metal ion Molecular orbitals Atomic orbitals in ligand ion
Molecular Orbital diagram for CoF6
3-
Do
Oh σ bonding
Clearly good σ donor ligand
Result in good M-L overlap
Strongly antibonding eg set
65. eg
t2g
t2g
ML6
s-only
ML6
s + π
Stabilization
(empty π-orbitals on ligands)
Do
D’o Do has increased
Case 1 (CN-, CO, C2H4)
empty π-orbitals on the ligands
M®L π-bonding (π-back bonding)
t2g (π)
t2g (π*)
eg
66. t2g
eg
t2g
ML6
s-only
ML6
s + π
Case 2 (Cl-, F-)
filled π-orbitals on the ligands
L® M π-bonding
(filled π-orbitals)
Stabilization
Destabilization
t2g (π)
eg
t2g (π*)
D’o
D Do o has decreased
68. π donor ligands lower
in E than t2g.
π acceptor ligands
higher in E than t2g.
Summary:
strong σ- or π-donor weak field ligands.
π-acceptors strong field ligands.
Or