Here's why:

* Plumbic refers to the lead(IV) ion (Pb⁴⁺). This ion is highly unstable and readily reduces to the more stable lead(II) ion (Pb²⁺).

* Dichromate refers to the Cr₂O₇²⁻ anion.

Therefore, a compound with the formula Pb(Cr₂O₇)₂ would be extremely unstable and likely decompose into lead(II) dichromate (PbCr₂O₇) and other products.

Instead of plumbic dichromate, you might be thinking of lead(II) dichromate (PbCr₂O₇). This is a stable compound.