We present a study of the electronic and magnetic properties of the multiple-decker sandwich nanowires ($CP-M$) composed of cyclopentadienyl (CP) rings and 3d transition metal atoms (M=Ti to Ni) using first-principles techniques. We demonstrate using Density Functional Theory that structural relaxation play an important role in determining the magnetic ground-state of the system. Notably, the computed magnetic moment is zero in $CP-Mn$, while in $CP-V$ a significant turn-up in magnetic moment is evidenced. Two compounds show a half-metallic ferromagnetic ground state $CP-Fe/Cr$ with a gap within minority/majority spin channel. In order to study the effect of electronic correlations upon the half-metallic ground states in $CP-Cr$, we introduce a simplified three-bands Hubbard model which is solved within the Variational Cluster Approach. We discuss the results as a function of size of the reference cluster and the strength of average Coulomb $U$ and exchange $J$ parameters. Our results demonstrate that for the range of studied parameters $U=2-4eV$ and $J=0.6-1.2eV$ the half-metallic character is not maintained in the presence of local Coulomb interactions.
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