From the well-known 1,3,5-triaza-phosphaadamantane (PTA, 1a), the novel N-allyl and N-benzyl tetrafuoroborate salts 1-allyl-1-azonia-3,5-diaza-7-phosphaadamantane (APTA(BF4), 1b) and 1-benzyl-1-azonia-3,5-diaza-7-phosphaadamantane (BzPTA(BF4), 1c) were obtained. These phosphines were then allowed to react with (Pt(μ-Cl)(C6F5)(tht))2 (tht = tetrahydrothiophene) affording the water soluble Pt(II) complexes trans-(PtCl(C6F5)(PTA)2) (2a) and its bis-cationic congeners trans-(PtCl(C6F5)(APTA)2)(BF4)2 (2b) and trans-(PtCl(C6F5)(BzPTA)2)(BF4)2 (2c). The compounds were fully characterized by multinuclear NMR, ESI-MS, elemental analysis and (for 2a) also by single crystal X-ray diffraction, which proved the trans configuration of the phosphine ligands. Furthermore, in order to evaluate the cytotoxic activities of all complexes the normal human dermal fibroblast (NHDF) cell culture were used. The antineoplastic activity of the investigated compounds was checked against the human lung carcinoma (A549), epithelioid cervix carcinoma (HeLa) and breast adenocarcinoma (MCF-7) cell cultures. Interactions between the complexes and human serum albumin (HSA) using fluorescence spectroscopy and circular dichroism spectroscopy (CD) were also investigated.