 Samarium forms a cyclopentadienide Sm ( C5H5 ) 3 and its chloroderivatives Sm ( C5H5 ) 2Cl and Sm ( C5H5 ) Cl2 . They are prepared by reacting samarium trichloride with NaC5H5 in tetrahydrofuran . Contrary to cyclopentadienides of most other lanthanides , in Sm ( C5H5 ) 3 some C5H5 rings bridge each other by forming ring vertexes η1 or edges η2 toward another neighboring samarium atom , thereby creating polymeric chains . The chloroderivative Sm ( C5H5 ) 2Cl has a dimer structure , which is more accurately expressed as ( η5 @-@ C5H5 ) 2Sm ( µ @-@ Cl ) 2 ( η5 @-@ C5H5 ) 2 . There , the chlorine bridges can be replaced , for instance , by iodine , hydrogen or nitrogen atoms or by CN groups . 
