The complex [NaYb(C5H5)(3)] (C5H5 = cyclopentadienyl), prepared from Yb(C5H5)(3) and sodium naphthalenide in tetrahydrofuran (thf) or from Yb(C5H5)(2) and Na(C5H5) in tetrahydrofuran and crystallized at ca, 400 degrees C (< 10(-4) mm Hg), is a three-dimensional polymer in which mu-eta(5):eta(5)-C5H5 groups link sodium and ytterbium (each metal having a pseudo-triangular arrangement of C5H5 ligands); Yb(C5H5)(2), similarly crystallized, has a polymeric chain structure with one terminal and two bridging C5H5 ligands per ytterbium, and there is eta(1)-bonding between adjacent chains.