_P.-Powell-auth.-Principles-of-Organometallic-Chemistry-Springer-Netherlands-1988

Diene complexes
Co 2 (C0) 8
(CeHe~Fe+BF4
o•c
flt
co+BF4
(C0)3
v (CO) 2150, 2102cm-1
r/?'\ Nu 1 (Nu 2 )- ~ 1
/ ( \ Nu 2_/ \_Nu
Co
(C0)3 Z-isomer
Nu 1 =H(NaBH 3 CN); Ph(PhMgBr); pyridine ;-CH(C0 2 Me)2 ; allyl (allyl SiMe 3); OMe(MeOH).
Nu1=H, Nl=-CH(C0 2 Me) 2
8.2.1 s-trans-Diene complexes
Normally the frontier orbitals of transition metal fragments such as Fe(C0) 3
favour bonding to the cis conformation of a diene. The fragments (17 5 -C 5 H 5 ) 2 M (M
= Zr, Hf) (p. 292), however. are well suited to 17 4 -bonding to trans dienes, as their
available orbitals alllie in the xz molecular plane.
Complexes Cp 2 Zr(17 4 -diene) can be prepared from Cp 2 ZrCI 2 and magnesium
butadiene (p. 9 3) in tetrahydrofuran, or by photolysis of Cp 2 ZrPh 2 with
butadiene. If the reactions are carried out at low temperature the kinetically
controlled trans-butadiene complex is produced, via and 17 2 -intermediate. On
warming to room tempera ture an equilibrium between the cis-17 4 - ( 5 5%) and
trans-17 4 - (45%) complexes becomes established.
\L
'~
zr<$'
/"-.
Cp Cp
s-trans-~4
Zr
c( "-cp
When the diene has terminal ( l, 4-) substituents the equilibrium !ies on the side of
the trans form, while with interna! (2, 3-) substituents the cis structure is
favoured. In both structures the zirconium atom adopts pseudo tetrahedral
geometry. In the cis complex the Zr-C distances to the terminal carbon atoms
are shorter than to the interna! carbons. This means that the structure
approaches the metallacyclopentene limit.
267