In the lanthanide series , how many members must there be ?
Lanthanide triflates can be convenient anhydrous precursors to many bis ( trimethylsilyl ) amides : Ln ( OTf ) 3 + 3 M ( hmds ) → Ln ( hmds ) 3 + 3 MOTf ( M = Li , Na , K ; Ln = La , Nd , Sm , Er ) However it is more common to see the preparation of lanthanide bis ( trimethylsilyl ) amides from anhydrous lanthanide chlorides , as these are cheaper .