The compound Na5Sn13 forms in the sodium-tin system in high yield following prolonged reaction of a quenched mixture at 280 degrees C: in a Ta container. The compound exhibits a complex three-dimensional structure (29 independent atoms) built mainly of 4-bonded tin atoms In interbonded pentagons (orthorhombic, space group Cmcm (No. 63); a = 8.979(1) Angstrom, b = 19.448(6) Angstrom, c = 50.43(2) Angstrom, Z = 16). According to Zintl-Klemm electron-counting formalisms, the compound has a closed-shell electronic configuration with the number of 3-bonded Sn-equal to the number of sodium cations. Property measurements show that the compound is diamagnetic.