# Why is Diiron nonacarbonyl so exceptional?

My textbooks says:

With exception of $\ce{Fe2(CO)9}$, all other metal carbonyls are soluble in hydrocarbon solvents.

Weller, M.; Overton, T.; Rourke, J.; Armstrong, F. Inorganic Chemistry, 6th ed. states:

The most striking exception among the common metal carbonyls is nonacarbonyl diiron(0), which has a very low vapour pressure and is insoluble in solvents with which it does not react.

Other metal carbonyl are well soluble in hydrocarbon solvents but not Diiron nonacarbonyl. Why? Why Diiron nonacarbonyl is so exceptional? Because it has low vapor pressure? But why?

There is no explaination regarding the insolubility of diiron nonacarbonyl. Is there any experiment conducted on this experiment to determine the explaination behind its insolubility of it is just mere observation?

## Update

@Orthocresol has said to draw comparison of diiron nonacarbonyl with $\ce{[Mn2(CO)10]}$ or $\ce{[Co2(CO)8]}$ according to their molecular weight. Based on this fact, I went on to research further on this topic. And from various handbooks and research notes I delved upon, I have drawn the following solubility comparison table. [Note: Only binuclear metal carbonyls were considered of form $\ce{[M2(CO)_x]}$ because they have same structure and thus should have same physical properties].

\begin{array}{c|c} \mathbf{Metal~carbonyl} & \mathbf{Solubility} \\\hline \ce{Mn2(CO)10} & \mathrm{ether, other~organic~solvents}\\ \ce{Tc2(CO)10} & \mathrm{ether,acetone} \\ \ce{Fe2(CO)9} & \mathrm{Insoluble~in~benzene,ether,petrol.Only~soluble~in~THF}\\ \ce{Rh2(CO)8} & \mathrm{organic~solvents}\\ \ce{Ir2(CO)8} & \mathrm{Ether,CCl_4}\\ \ce{Co2(CO)8} & \mathrm{petrol,benzene,alcohol}\end{array}

Also, a table of comparison for the other two iron carbonyls for a reference.

\begin{array}{c|c} \mathbf{Iron~carbonyl} & \mathbf{Solubility} \\\hline \ce{Fe(CO)5} & \mathrm{soluble~in~all~organic~solvents~like~ether,petroleum.Insoluble~in~water}\\ \ce{Fe3(CO)12} & \mathrm{Insoluble~in~water. Soluble~in~non~polar~organic~solvents. } \end{array}

We can observe that there is a drastic anomaly in case of solubility of diiron nonacarbonyl for both the tables. Any explanation to this?

• It's solid not liquid like Fe(CO)5 therefore it has low vapor pressure. – Mithoron Nov 26 '16 at 17:34
• @Mithoron Ok, so why it exist as solid whereas other iron carbonyls are liquids? – Nilay Ghosh Nov 27 '16 at 4:29
• Well, almost twice as big molecule make wonders ;) – Mithoron Nov 27 '16 at 15:48
• If it's solely based on molecular weight, then I suppose a good comparison would be $\ce{[Mn2(CO)10]}$ or $\ce{[Co2(CO)8]}$. – orthocresol Aug 27 '17 at 16:33
• @orthocresol. I added a comparison table. – Nilay Ghosh Nov 20 '17 at 7:26