She convinced him that he was wrong.
In this sentence, would we describe 'that he was wrong' as a direct object and 'him' as an indirect object? I would think that convinced acts upon 'him'; however, that would make the 'that- clause' a complement of the verb 'convinced'. If this is the case, is this an example of an omitted preposition (see the example below)?
She convinced him [about] that he was wrong.
We often see cases of omitted prepositions with gerunds (see below), so I believe this is a possible explanation.
They rowed the boat [by] using all of their strength.
Keep in mind that this is according to traditional grammar. Modern grammar probably has a different stance, but I am not educated enough to know it.