Yttrium compounds can serve as host lattices for doping with different lanthanide cations. Diarylmercurials are often used to synthesize lanthanide organometallic complexes. Cerium is the second element of the lanthanide series. This is an effect of the lanthanide contraction&colon; a similar actinide contraction also exists. Lanthanum is the first element and prototype of the lanthanide series. Doping these with other lanthanide elements produces phosphors. Thulium is the least abundant lanthanide on earth except for the radioactive promethium. All of these forms are strongly electropositive and thus lanthanide ions are hard Lewis acids. Mass cytometry overcomes the fluorescent labeling limit by utilizing lanthanide isotopes attached to antibodies. Being a typical member of the lanthanide series, samarium usually assumes the oxidation state +3. Gadolinium(III), like most lanthanide ions, forms complexes with high coordination numbers. Oxalic acid is an important reagent in lanthanide chemistry. EDTA was used in separation of the lanthanide metals by ion-exchange chromatography. Many of these features make lanthanide complexes effective catalysts. Neodymium belongs to the lanthanide series and is a rare-earth element.