I don’t know what
Каді́й- (Kadíj-) represents. Structurally, there is
ка́дій m anim (kádij,
noun), but I don’t know what it means, and there is also
ка́діїв (kádijiv,
adjective), in case
Вікісловника etymology is indeed synchronic, but I don’t know what that means, either. Semantically, there exist
кади́ло n inan (kadýlo,
“censer”,
noun) and
кади́ти impf (kadýty,
“?burn incense, ?fumigate”,
verb), which are both related to
чад m inan (čad,
“smoke, fumes”,
noun), to which family of words we may expect
Кадіївка to be related, given that “There were thirty coal mines in
Kadiivka Raion.” (according to
w:en:Kadiivka Raion#Economy, which cites «
З 30 шахт Кадіївського району 15 шахт було введено в експлуатацію після революції.»). I note that entries for the Ukrainian suffixes
-ів (-iv) and
-ка (-ka) both exist, whereas
-івка (-ivka), whose entry is as yet uncreated, occurs in very many Ukrainian placenames.