In metallic sulphide the sulphur is in ionic form and can donate electron forming co-ordinate complex. Whereas in thiourea ,sulphur forms a ligand in it and is not in free ionic form. So it cannot displace sulphur from metallic sulphide.

Metals such as zinc often do not readily form precipitates with thiourea because the interaction between zinc ions and thiourea is relatively weak compared to other transition metals. Thiourea acts primarily as a sulfur source in metal sulfide synthesis, but it does not immediately release sulfide ions (S²⁻). Instead, thiourea requires specific conditions like heating or alkaline pH to undergo hydrolysis or decomposition, which then releases sulfide ions that can react with metal ions to form insoluble metal sulfides. In the case of zinc nitrate mixed with thiourea under normal conditions, the thiourea remains largely intact without releasing free sulfide ions, so the zinc stays dissolved and no precipitate forms. Additionally, zinc-thiourea complexes that do form tend to be more labile and soluble, stabilizing zinc ions in solution rather than allowing them to precipitate as zinc sulfide. Therefore, to promote the formation of ZnS precipitate, reaction conditions must favor the breakdown of thiourea—usually involving elevated temperatures and alkaline environments. These factors explain why zinc nitrate and thiourea mixtures often do not yield an immediate precipitate, as opposed to other metal ions with stronger complexation and faster sulfide release mechanisms.