We present abundances of Mn, Cu, Zn and various light and heavy elements for a sample of barium and normal giant stars, and present correlations between abundances contributed to different degrees by the weak-s, main-s, and r-processes of neutron capture, between Fe-peak elements and heavy elements. Data from the literature are also considered in order to better study the abundance pattern of peculiar stars. The stellar spectra were observed with FEROS/ESO, and the stellar atmospheric parameters of the 8 barium giant stars and 6 normal giants analyzed by us lie in the range 4300 < T_eff/K < 5300, -0.7 < [Fe/H] <= 0.12 and 1.5 <= log g < 2.9. Carbon and nitrogen abundances were derived by the spectral synthesis of molecular bands of C_2, CH and CN. For all the other elements, atomic lines were used to perform the spectral synthesis. A very large scatter was found when data from the literature were considered, mainly for the Mn abundances. We found that [Zn/Fe] correlates well with the heavy element excesses, its abundance clearly increasing as the heavy element excesses increase, a trend not shown by the [Cu/Fe] and [Mn/Fe] ratios. Also, the excess of Mn, Cu and Zn relative to heavy elements usually show an increasing trend toward higher metallicities. Our results suggest that a larger fraction of the synthesis of Zn is owed to massive stars than is the case of Cu, and that the contribution of the main-s process to the synthesis of both elements is small. We also conclude that Mn is mostly synthesized by SN Ia, and that a non-negligible fraction of the synthesis of Mn, Cu and Zn is owed to the weak s-process.