New manganese(II) complexes with Mn2Cl2 and more rare Mn2O2 cores were obtained based on the diisopropyl(pyridin-2-yl)phosphine oxide (L2) and diethyl(pyridin-2-yl)phosphine oxide (L3). The crystal structures and sample purity of the compounds were determined by single-crystal and powder X-ray diffraction. The luminescence, the electrocatalytic activity in hydrogen evolution reaction, and the cytotoxicity against the M−HeLa and HuTu80 cancer cell lines and Chang liver normal cell lines were investigated. It was found that manganese(II) complex 3 with the Mn2O2 core was the effective catalyst in the hydrogen evolution reaction and exhibited high cytotoxicity against the cancer HuTu80 cell line.