A one pot reaction of Li2{1,4-(Me3Si) 2C8H6}, LnCl3, and K{CH(PPh 2NSiMe3)2} leads to the 1,4-bis(trimethylsilyl) cyclooctatetraene bis(phosphinimino)methanide complexes of yttrium and erbium, [{CH(PPh2NSiMe3)2}Ln(η8-{1,4- (Me3Si)2C8H6})] (Ln = Y, Er). Both complexes have been characterized by single crystal X-ray diffraction. The solid state structures show that the two bulky ligands cause a steric crowding around the lanthanide atom. As a result of this steric crowding both ligands are asymmetrically attached to the lanthanide atom.