Why is Zeise's salt considered the first synthetic organometallic compound?

Zeise's salt, with the chemical formula $\text{K}[{\text{PtCl}}_3({\text{C}}_2{\text{H}}_4)]$, is considered the first synthetic organometallic compound because it was the first such substance to be prepared in a laboratory setting, a feat achieved by the Danish pharmacist William C. Zeise in 1827.1

The compound qualifies as organometallic because it features an ethylene molecule (${\text{H}}_2\text{C}={\text{CH}}_2$) attached to a central platinum atom through both of its carbon atoms. While Zeise could not determine the precise structure of the salt at the time of its discovery, modern chemistry recognizes that the alkene ligand bonds to the metal center through both electron donation and acceptance. This direct attachment of carbon atoms to a metal atom is the defining characteristic of an organometallic compound, distinguishing it from other metal complexes.123