Download Alkene complexes: Dewar-Chatt Bonding Model

Alkene complexes: Dewar-Chatt-Duncanson Bonding Model
π
(1)
(2)
π*
C
C
M
M
C
C
σ-donation
π-acceptance
• Synergic Bonding
• Both σ-donation and π-acceptance weakens the CC bond
Alkyne complexes: Dewar-Chatt-Duncanson Bonding Model
(1)
π
(2)
π*
C
C
M
M
C
C
πII
σII
(3)
π
(4)
π*
C
C
M
M
C
π
C
δ
• Synergic Bonding
• All four possible bonding interactions weaken the CC bond
• Alkynes can be 2 or 4 electron donors, depending on whether the π┴
interaction is significant for a given complex
Typical Ligands (Alkenes)
- Alkene versus Metallacyclopropane Complex
Cl
Cl
Ph3P
Pt
Ph3P
Pt
Cl
Ph3P
II
Pt Alkene
137.5 pm
134 pm
Pt
Ph3P
143.0 pm
PtII Metallacyclopropane
Pt0 Alkene
153 pm
- Synthesis of Alkene Complexes
Substitution : [PtCl4]2- + C2H4
[PtCl3(C2H4)]- + Cl-
Reduction of a Higher Valent Metal in the Presence of an Alkene:
2 RhCl3 +
Cl
Cl
Rh
+ 2 EtOH
O
Rh
+ 4 HCl + 2
H
From Alkyl or Allyl Complexes:
(OC)5Mn
Cp2Ta
H+
β-H elim.
(OC)5Mn
Cp2Ta
+
H
+
Typical Ligands (Alkynes)
- Alkyne versus Metallacyclopropene Complex
R3N
Pt
CC = 124 pm
ν(CC) = 2028 cm-1
Cl
II
Ph
Cl tBu
Pt Alkene
tBu
Ph3P
Ph3P
Pt Alkyne
CC = 132 pm
ν(CC) = 1750 cm-1
Pt
Pt
Ph3P
Ph
0
Ph
Ph3P
II
Ph
Pt Metallacyclopropene
Alkyne Coupling
Connelly + Emslie, Dalton Trans, 2003, 2835