Charles Ellicott Commentary 2 Chronicles 11:22

"And Rehoboam appointed Abijah the son of Maacah to be chief, [even] the prince among his brethren; for [he was minded] to make him king." — 2 Chronicles 11:22 (ASV)

And Rehoboam made Abijah the son of Maachah ... brethren. — Rather, And Rehoboam appointed Abijah the son of Maachah for head — that is, for prince (nagîd) — among his brethren. The expression “head” is explained by the following clause.

For he thought to make him king. — This gives the sense of the brief Hebrew phrase, “for—for making him king.” In making Abijah heir to the throne, it does not appear that Rehoboam infringed the law of Deuteronomy 21:15-17, as the Speaker’s Commentary suggests. The right of the firstborn was only a double share of a man’s property (Compare to 1 Kings 1:35 for a precedent).