Here's why:

* Object complements provide more information about the direct object. They usually describe or modify the object in some way. They often come after a direct object and are linked to it by a linking verb (like "be," "seem," "become").

* Scientists are people, and people are not typically used as object complements. They are the subjects or objects of verbs, not descriptions of other objects.

Example:

* Incorrect: "They appointed her a scientist." (This doesn't make sense because "scientist" is not describing the object "her".)

Correct:

* "They appointed her to the position of scientist." (Here, "to the position of scientist" describes the object "her.")

Instead of using a scientist as an object complement, you can:

* Use a noun phrase to describe the object: "They appointed her the lead researcher."

* Use an adjective to describe the object: "They found her qualified."

* Use a prepositional phrase to describe the object: "They assigned her to the project."