The conjugated diruthenium bisalkylidene complex [(Cy
3P)
2Cl
2Ru]
2(
![](/images/entities/mgr.gif)
-CHCH=CHCH) (
1,an inseparable 10:1 mixture of
E and
Z alkene isomers
1E and
1Z) and the separate
E and
Z isomers of the monoruthenium alkylidene complexes (Cy
3P)
2Cl
2Ru=CHCH=CHCH=CH
2(
2E and
2Z) and (Cy
3P)
2Cl
2Ru=CHCH=CHCH
3 (
3E and
3Z) have each been synthesizedthrough the stoichiometric reaction of olefin metathesis catalysts. Each pair of isomersundergo
Z-E isomerization, though the isomerization rates and their dependencies on excessalkene and PCy
3 are very different. Isomerization in
1 is the fastest, with
H![](/images/entities/thermod.gif)
= 100 ± 5kJ/mol and
S![](/images/entities/thermod.gif)
= 67 ± 3 J/(mol K) (
1Z to
1E) or 49 ± 3 J/(mol K) (
1E to
1Z) as determinedby dynamic NMR, giving
t1/2 = 11 s for the approach to equilibrium at 25
![](/images/entities/deg.gif)
C. The rate ofthis isomerization shows no phosphine dependence. Isomerizations of
2Z and
3Z are slower(
t1/2 = 5-20 h), but the former is promoted by PCy
3, while the latter is inhibited. In thepresence of (
Z)-1,3,5-hexatriene, the isomerization of
2Z is much slower, while theisomerization of
3Z is accelerated by (
Z)
-1,3-pentadiene. Density functional calculations wereperformed to help interpret these results and elucidate which of several possible isomerizationmechanisms operates in each case.