It was an agreement the details of which could not be altered.这里面的of 是介词前置吗?

It was an agreement(that) the details of which could not be altered.这里面的of 不是介词前置不是介词前置，哪是什么呢，麻烦请仔细给说说the details of which,which这里指an agreement，即the details of an agreement, 没有介词前置