[J]

Hi,

I am studying pi-arene ligands and there bonding to transition metals (in this case ruthenium)

I have been studying the "piano stool" structure of the compound (η6-p-cymene)RuCl2(PPh3) and was wondering the reasons why the Ru-C bonds are longer than in e.g. [(η6-p-cymene)RuCl2]2.

I'm sure it has something to do with the PPh3 ligand but I am unable to justify this.

Any help on this would be greatly appreciated.

J

[renge ishyo]

The structure of the [(η6-p-cymene)RuCl2]2 dimer arises from the chlorines pulling the electron density away from the central ruthenium atom. When it does this it creates a partial positive charge on the central ruthenium atom. This allows the pi bond system of the aromatic ring containing molecule to establish a bond with the partially positive Ruthenium atom by donating its electron density onto the positive ruthenium atom. The polarization of the Ruthenium by the two chlorines is so great that this arrangement also allows for an additional weak attachment to an additional chlorine from a second copy of the molecule (η6-p-cymene)RuCl2. This forms the dimer structure (see the wiki for an image: http://en.wikipedia.org/wiki/(Cymene)ruthenium_dichloride_dimer). Note that the bonds are shorter and stronger in general the more strongly polarized the ruthenium central atom is. This ruthenium atom is attached to three chlorines in the dimer which are strongly electronegative.

So what changes when you add PPh3? Well, first off PPh3 is a Lewis Base that can compete with chlorine for binding to the central ruthenium atom. As it donates electron density into the Ruthenium it also breaks off the  coordination of the additional chlorine from the second molecule that was attached to the Ruthenium to make the dimer. This breaks apart the dimer to form the monomer compound, (η6-p-cymene)RuCl2(PPh3). The big and bulky PPh3 molecule assures that the monomers cannot come together once again to form a dimer complex because the additional chlorine cannot get close enough to the ruthenium to donate in. However, in doing so the PPh3 molecule *donates* electron density onto the Ruthenium central atom as opposed to pulling it away from the ruthenium as the chlorine atoms did. This decreases the partial positive charge on the ruthenium atom. Now it is attached to two electronegative chlorines and one partially electropostive PPh3 molecule. A smaller partially positive charge on the ruthenium atom translates into weaker bonds that are attached to it which are now longer in length than they were before.