(Bamidbar 7:2) "And the chiefs of Israel presented (their offerings"): I might think that they were common people appointed (to their position); it is, therefore, written "the heads of their fathers' house." And not only the heads of their fathers' house, but also "the chiefs of the tribes." And what is the thrust of "the heads of their fathers' house"? Chiefs the sons of chiefs. ("the heads of their fathers' house) were they" — they who were appointed over them in Egypt, viz. (Shemot 5:14) "And the officers of the children of Israel were beaten, etc." (Ibid. 3) "And they presented their offerings before the L-rd, six wagons tzav." "tzav" is "opulent" — they were lacking nothing.
Rebbi says: "tzav" is "covered," as gluskaoth are. And though there is no proof for this, it is intimated in (Isaiah 66:20) "And they will bring all your brothers from all the nations as an offering to the L-rd, on horses, and in chariots, and in litters ("tzavim") and on mules and on dromedaries." "six wagons tzav": I might think, a wagon for each one (of six, who donated them); it is, therefore, written "a wagon for every two chiefs."
I might think an ox for two chiefs; it is, therefore, written "and an ox for each." They came and stood before the mishkan, but Moses would not accept them until it was told to him by the Holy (Spirit): Take it from them. Their minds are at one with that of the Most High. R. Nathan says: What did the chiefs see to come first with their offerings here, but not in the offerings for the work of the mishkan? (In that instance) they said: Let Israel offer what they will and we will make up what is missing.
When they saw that the congregation had completed everything, viz. (Shemot 36:7) "And what they had was enough for all of the work and more," they said: What is left for us to do? And the chiefs brought the onyx stones (viz. Ibid. 35:27) Therefore, they brought their offerings first here.