These were the sons of Reuben the firstborn of Israel. Though he was the firstborn, his birthright was given to the sons of Joseph son of Israel, because Reuben defiled his father’s bed. So he is not reckoned according to birthright.
We have here an extract out of the genealogies, I. Of the tribe of Reuben, where we have, 1. The reason why this tribe is thus postponed. It is confessed that Reuben was the first-born of Israel, and, upon that account, might challenge the precedency; but he forfeited his birthright by defiling his father's concubine, and was, for that, sentenced not to excel, Gen 49:4.
Commenting on 1 Chronicles 5:1-17
Now the sons of Reuben, the firstborn of Israel,.... Are as follow in Ch1 5:3 where the account begins; for what comes between this and that is in a parenthesis: for he was the firstborn; of Jacob by his wife Leah; that must be owned, and Jacob allows it, Gen 49:3 and yet the genealogy in this book begins not with him, as might on...
Now the sons of Reuben the firstborn of Israel, (for he [was] the firstborn; but, forasmuch as he defiled his father's bed, his birthright was given unto the sons of Joseph the son of Israel: and the genealogy is not to be reckoned after the birthright. (a) Because they were made two tribes, they had a double portion.