There was nothing in the ark except the two tablets that Moses put there at Horeb, where the LORD made a covenant with the people of Israel, when they came out of Egypt.
This agrees with what we had Kg1 8:2, etc., where an account was given of the solemn introduction of the ark into the new-erected temple. 1. There needed no great solemnity for the bringing in of the dedicated things, Ch2 5:1.
Commenting on 2 Chronicles 5:1-10
[There was] nothing in the ark save the two tables which Moses put [therein] at Horeb, when the LORD made [a covenant] with the children of Israel, when they came out of Egypt. (d) For Aaron's rod and manna were taken from there before it was brought to this place.
There was nothing in the ark save - The Chaldee paraphrases thus: "There was nothing put in the ark but the two tables which Moses placed there, after the first had been broken on account of the calf which they made in Horeb, and the two other tables had been confirmed which were written with writing expressed in the Ten Words."