As per IUPAC convention, transition metal atom have incomplete d-subshell.

6d-series includes all elements from $\mathrm{Ac}(\mathrm{Z}=89)$ to $\mathrm{Cn}(\mathrm{Z}=112)$.

These elements are placed in group 3 to 12 in modern periodic table.

In these ( $\mathrm{n}-1$ ) d-orbital is filled successively.