The electronic structures of trichloro[(1,2,3,3a,7a-eta)-1H-inden-1-yl]-hafnium and bis(cyclopentadienyl) hafnium dichloride have been investigated by HeI and HeII UV photoelectron spectroscopy and DFT/OVGF calculations. The chlorine lone pair ionization energies are reduced considerably on going from HfCl4 to metallocene hafnium chlorides. The electronic structure results have been related to the mechanism of olefin polymerization catalyzed by hafnocenes. (C) 2011 Elsevier B.V. All rights reserved.