Local spin-density-functional calculations are performed to study the geometric and electronic structure of PbNa6 clusters. Three possible isomeric structures are examined. Both the electronic and the geometric factors are shown to be important to the stabilization of the cluster and the most stable structure is found to be a tricapped tetrahedron. The electronic structure of this cluster indicates that the metallic bonding interaction of delocalized electrons is responsible for the exceptional stability of the cluster.