The objective of this study is to design and synthesize substituted η6-chromium(0) tricarbonyl metal complexes carryingo-carborane units as potential boron neutron capture therapy (BNCT) agents. In this study, 1,2-diphenyl-o-carborane (1) units were used as starting materials to generate biologically active species. We investigated how the structural changes of1substituted with chromium(0) tricarbonyl affect the biological properties, and 1-(Phenyl-η6-chromium(0) tricarbonyl)-2-phenyl-o-carborane (2) and 1,2-bis(phenyl-η6-chromium(0) tricarbonyl)-o-carborane (3) species were produced in moderate yields. The molecular structures of compounds1–3were identified and established by infrared (IR);1H,11B, and13C nuclear magnetic resonance (NMR) and X-ray crystallography analyses. Crystal structures of 1,2-diphenyl-o-carborane and the corresponding chromium complexes1,2, and3were obtained. In an in vitro study using B16 and CT26 cancer cells containing the triphenyl-o-carboranyl chromium(0) complexesPh3C2BCr2andPh3C2BCr3, which we reported previously, compounds2and3accumulated at higher levels than compoundsPh3C2BCr2andPh3C2BCr3. However, the phenylatedo-carboranyl chromium complexes have been found to be more cytotoxic thanp-boronophenylalanine (BPA).