Building blocks and formation thermodynamics of α-Keggin-type [PW12O40]3− anion

► We study theoretically formation mechanism of α-Keggin-type [PW12O40]3− anion. ► We obtain various building units in the polymerization processes of PW12 anion. ► The starting reaction has been determined through transition state search. ► Heteroatom (P) has been introduced in the heterotrimer [PW2O9]− at the second step. ► Competition between hetero- and iso-polytungstate polymerizations is essential.