Help with the passive voice

Hi, I need some help with the passive voice. In these examples:

Active: People think that George gave them some money.

George: It is thought that George gave them some money.

George is thought to have given them some money.

Them: It is thought that they were given some money by George.

They are thought to have been given some money by George.

Some money: It is thought that some money was given to them by George

Some money is thought to have been given to them by George.

After “to”, an infinitive form is used… could anyone please tell me what is the grammar rule to this?.
Thanks a lot.