Q. about the Cossee-Arlman mechanism in alkene polymerization

[anthemrider22]

Starting out with TiCl₄ and triethylaluminum, I understand how the triethylaluminum alkylates the TiCl₄ and creates a polymeric TiCl₃, but then an ethene molecule coordinates with a vacant site on the alkylated TiCl₃? I thought titanium's highest oxidation state was +4?

 

[chemisttree]

That's what makes it catalytic. It let's go again.


However... think about this. What would happen to the oxidation state of Ti⁺⁴ if you added electron density to it (in the form of a double bond)? Would it go up or down?

 

[anthemrider22]

The oxidation state would go down right? When the ethene molecule coordinates with the alkylated TiCl₃ does it occupy an empty d orbital on the titanium atom? I suppose that would create an unstable intermediate and open the door for the olefin insertion step to create another vacant site on the titanium atom. Thank you for your help.