PURPOSE: To prepare new platinum complexes useful for anti-tumor agents.

CONSTITUTION: This is the new compounds represented by the formula (R is an aromatic, aliphatic or alicyclic organic compound residue; each of L and L' is an ammine, primary monoamine, secondary monoamine, pyridine type amine or dimethylformamide), for example, cis-((Pt(cyclobutylamine)(NH3))2(cis, cis,cis,cis-1,2,3,4-cyclopentyltetracarboxylic group)). The compound represented by the formula is prepared by allowing a compound represented by the formula; cis-(PtL2I2) to react with an acid, allowing the resultant oligomer to react with an amine, allowing the obtained compound represented by the formula cis-(PtLL' I2) to react with a silver salt and allowing the resultant compound to react with a tetrabasic carboxylic acid salt.