  The magnetic properties of the two isostructural molecule-based magnets,
Ni(NCS)$_{2}$(thiourea)$_{2}$, $S$ = 1, [thiourea = SC(NH$_2$)$_2$] and
Co(NCS)$_{2}$(thiourea)$_{2}$, $S$ = 3/2, are characterised using several
techniques in order to rationalise their relationship with structural
parameters and ascertain magnetic changes caused by substitution of the spin.
Zero-field heat capacity and muon-spin relaxation measurements reveal
low-temperature long-range ordering in both compounds, in addition to
Ising-like ($D < 0$) single-ion anisotropy ($D_{\rm{Co}} \sim$ -100 K,
$D_{\rm{Ni}} \sim$ -10 K). Crystal and electronic structure, combined with
DC-field magnetometry, affirm highly quasi-one-dimensional behaviour, with
ferromagnetic intrachain exchange interactions $J_{\rm{Co}}\approx+4$ K and
$J_{\rm{Ni}}\sim+100$ K and weak antiferromagnetic interchain exchange, on the
order of $J'$ $\sim-0.1$ K. Electron charge and spin-density mapping reveals
through-space exchange as a mechanism to explain the large discrepancy in
$J$-values despite, from a structural perspective, the highly similar exchange
pathways in both materials. Both species can be compared to the similar
compounds $M$Cl$_2$(thiourea)$_4$, $M$ = Ni(II) (DTN) and Co(II) (DTC), where
DTN is know to harbour two magnetic field-induced quantum critical points.
Direct comparison of DTN and DTC with the compounds studied here shows that
substituting the halide Cl$^-$ ion, for the NCS$^-$ ion, results in a dramatic
change in both the structural and magnetic properties.

Strongly Correlated Electrons