Why do diplumbenes (Pb=Pb double bonded compounds) break apart in hydrocarbon solution?

In group 14, the double bond becomes increasingly weaker down the group. Our lecturer told us that many distannenes ($\ce{>Sn=Sn<}$) and almost all diplumbenes ($\ce{>Pb=Pb<}$) dissociate into stannylene ($\ce{R2Sn:}$) and plymbylenes ($\ce{R2Pb:}$) when dissolved in hydrocarbon solution.


The weak nature of $\ce{Sn=Sn}$ and $\ce{Pb=Pb}$ bond has been explained by the $\mathrm{6s-6p}$ energy gap, which makes the $\mathrm{p-p}$ pi-bond unfavourable; instead the $\ce{sp^2}$ orbitals of each plumbylene moiety overlaps with the empty p orbitals of the other. Mixing between $\sigma^\*$ and $\pi$ has also been used to explain the weak double bond for distannenes and diplumbenes.


If the double bond is really weak, then why don't these compounds exist as plumbylene monomers in solid state as well? How does dissolving in *hydrocarbon* make the plumbylene monomer more stable than the dimer diplumbene?


Hydrocarbons can only provide weak van der Waals interactions, so how can they make a large difference to the equilibrium between dimer and monomer?


