Class Notes (1,100,000)
CA (650,000)
UVic (3,000)
CHEM (200)
Lecture 6

CHEM 324 Lecture Notes - Lecture 6: Sigma Bond, Valence Electron, Lone Pair


Department
Chemistry
Course Code
CHEM 324
Professor
Robin Hicks
Lecture
6

This preview shows pages 1-3. to view the full 21 pages of the document.
5B-1 The 18 electron “rule”
!
!
!
!
!
!
!
don't tend to be so antibonding that !
they break bonds!
!
!
!
!
two flavours:!
1. d-electrons!
2. sigma bonds !
from ligands!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
Number of M-L bonding MOs= number of ligands!
In most cases 18e allows population of bonding/weakly anti bonding d-based orbitals

Only pages 1-3 are available for preview. Some parts have been intentionally blurred.

5B-2 The 18 electron “rule”
Stable transition metal complexes tend to have a total of 18 valence electrons associated with
the transition metal.
H&S 20.4 & 24.3
Exceptions:
1. complexes of early transition metals (not enough d-electrons)
2. Late transition metals (too many d-electrons), particularly with weak sigma donors (more on
this in a bit)
3. Square planar complexes these obey the 16 electron rule
Note that we use the “ionic” model of M-L bonding, where formally anionic ligands are two-
electron donors (this is the model where we emphasize polar nature of the M-L bond). There is
an alternative model for counting (mentioned in 24.3) in which M-L bonds are broken
homolytically. We don’t use this (but its valid).
d-electrons on the metal 1.
Electrons involved in M-L sigma bonding 2.
Eg. TiCl4(THF)21.
Eg. Ni(H2O)6]2+1.
Heterolytic M-L -> M : L 1.
Homolytic M-L--> M .L 2.

Only pages 1-3 are available for preview. Some parts have been intentionally blurred.

5B-3 Pi bonding in Octahedral complexes H&S 20.4
a1g + eg+ t1u a1g + eg+ t1u
t2g
As before: L is a σdonor L is a πdonor
(and still a σdonor)
You're Reading a Preview

Unlock to view full version