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Bonding in Molecules
Lecture 4
d orbitals
Bonding in Hypervalent Molecules
XeF2 (‘hypervalent’ = 10 electrons on Xe)
Possible explanations?
sp3d hybridisation?
Ionic resonance forms?
Total bond order 4
(2 per C=O bond)
Total bond order = 1.0
(0.5 per Xe-F bond)
d orbitals improve bonding, but we can explain the stability of XeF2 without them
They do not participate to the extent implied by a formal sp3d hybridisation model
unrealistic
more realistic
SF6
sp3d2 hybrids?
ionic resonance?
S-F bond order = 4/6
MO diagram for octahedral AH6
E
E
E
E
Total bond order = 4
E
=4/6 per A-H bond
Non-bonding 1eg
electrons localised
entirely on H (so
ligand has to be
electronegative)
allowing nd to participate in bonding
Total bond order = 6
E
=1 per A-H bond
1eg electrons
delocalised
over A and H
d orbitals improve bonding, but we can explain the stability of SF6 without them
They do not participate to the extent implied by a formal sp3d2 hybridisation model
[C(AuPR3)6]2+ (an analogue of [CH6]2+): 8 valence electrons
sp3d2 hybrids?
ionic resonance?
Very unlikely!
1eg filled
1eg empty
Bonding in Transition Metal Complexes
Main group:
ns < np < nd
Transition metal:
nd < (n+1)s < (n+1)p
S Cr
metallic radius for
Co = 125 pm
(••••••••)
rmax 145 pm
4πr2ψ2
rmax 40 pm
Distance from nucleus
Overlap with 3d is small (bonding dominated by overlap with 4s)
Overlap improves down group so bond strengths increase 3d < 4d < 5d
Contrast main group where bond strengths decrease 2p < 3p < 4p < 5p
Overlap as a function of orbital size:
A
SAA
A
Cr Mo W
O S Se
orbital size
σ-only MO diagram for ML6
Symmetry analysis
σ ‘lone pairs’ on ligands
σ-only MO diagram for ML6
E
σ-only MO diagram for ML6
E
σ-only MO diagram for ML6
E
σ-only MO diagram for ML6
E
σ-only MO diagram for ML6
E
σ-only MO diagram for ML6
Note a1g very strongly
antibonding.
So unlike bare atoms,
complexes never have
electrons in ‘s’ orbitals:
Cr is 4s13d5 but
Cr in Cr(CO)6 is
(t2g)6(eg)0(a1g)0
i.e. 4s03d6
E
σ-only MO diagram for ML6
6 non-bonding electrons
12 bonding electrons
(‘18 electron’ rule)
E
σ-only MO diagram for ML6
E
Region we focus
on in crystal/ligand
field theory (∆O)
High-spin/Low-spin (see Transition metals lecture course)
High-spin if ∆O < spin pairing energy
Low-spin is ∆O > spin pairing energy
Typical values:
1st row TM
P = 15000-25000 cm-1
2nd/3rd row TM
P smaller
What controls the magnitude of ∆o?
the spectrochemical series
∆o
∆E
1
∆o ∝
∆E