  Polynuclear magnetic molecules often present dense transmission spectra with
many overlapping conduction spin channels. Single-metal complexes display a
sparser density of states, which in the presence of a fixed external magnetic
field makes them interesting candidates for spin filtering. Here we perform a
DFT study of a family of bis- and tris-dithiolate vanadium complexes sandwiched
between Au(111) electrodes and demonstrate that [VO(C$_3$S$_4$O)$_2$]$^{2-}$
can behave as a dual spin filter. This means that an external stimulus can
switch between the selective transmission of spin-up and spin-down carriers. By
using an electrostatic gate as external stimulus we show that the onset for the
spin-up conductance is at a voltage V$_g$ = -0.51 V but a small shift to V$_g$
= -0.63 V is capable of activating spin-down transport. For both cases, we
estimate a large low-bias conductance (approx. 2 {\mu}S at Vbias < 50 mV) with
excellent spin selectivity (> 99.5%). We conclude by commenting on the general
molecular requirements for the chemical design of further such spintronics
components.
