CoCl6 does not exist.
Common oxidation states of cobalt include +2 and +3, so two different chlorides are possible: CoCl2 and CoCl3 but the last one is rare.
(CoF3 (Co(III)-fluoride) is a better possibillity to prove this oxidation state)
Maximum oxidation state is +5, though Cobalt can easily 'coordinate' with in total 6 coordinative groups (=CG) and negative ions: Co(CG)4Cl2 (a coordinative group migh be a < -N| >-atom group of ammonia, ethylenediamine or haemoglobin)
The compound formed between chromium(VI) and chlorine is chromium(VI) chloride, with the chemical formula CrCl6.
The chemical formula for uranium(VI) iodide is UI6.
CrS3 is the formula of chromium(VI) sulfide
That would be chromium(IV) selenide with the formula CrSe2
Cobalt tetraoxosulphate (vi) ion
The compound formed between chromium(VI) and chlorine is chromium(VI) chloride, with the chemical formula CrCl6.
The chemical formula for uranium(VI) iodide is UI6.
CrS3 is the formula of chromium(VI) sulfide
That would be chromium(IV) selenide with the formula CrSe2
it is same as ammonium sulphate, (NH4)2SO4
Cobalt tetraoxosulphate (vi) ion
The chemical formula for manganese(IV) iodide is MnI4.
The chemical forrmula of potassium heptaoxodichromate(VI) is K2Cr2O7. The systematic IUPAC name is potassium dichromate (VI).
A name for this compound would be cobalt dinitride. Co forms nitrides, CoN, Co2N and Co2N3, the only one with ionic characteristics is Co2N3. I do not think CoN2 has been characterised. Certainly if it was it would be misleading to call it Cobalt(VI) nitride.
The chemical formula for chromium VI phosphate is Cr(PO4)2.
Cobalt tetraoxosulphate (vi) ion
CrO3 is Chromium(VI) oxide also called Chromium trioxide