The ground-state geometries, electronic structures, and vibrational frequencies of alkali metal-B-5(-) MB5 (M = Li, Na, K, Rb, and Cs) and alkaline earth metal-B-5(-) MB5+ (M = Be, Mg, Ca, and Sr) clusters were investigated using ab initio self-consistent field and density functional theory (DFT) methods. Calculation results show that planar B-5(-) anion can coordinate with the metal atom to form metal-polyboron MB5 and MB5+ species. On the basis of the molecular orbital (MO) analysis and nucleus-independent chemical shifts (NICS), we revealed that the planar B-5(-) anion exhibits characteristic of pi-aromaticity with two delocalized pi electrons and maintains its structural and electronic integrity inside the five MB5 and four MB5+ clusters.