  Understanding the effect of chemical composition on the strength of magnetic
interactions is key to the design of magnets with stronger exchange
interactions. The magnetic divalent first-row transition metal (TM)
thiocyanates are a class of chemically simple layered molecular frameworks.
Here, we report two new members of the family, manganese (II) thiocyanate,
Mn(NCS)$_{2}$, and iron (II) thiocyanate, Fe(NCS)$_{2}$. Using magnetic
susceptibility measurements on these materials and on cobalt (II) thiocyanate
and nickel (II) thiocyanate, Co(NCS)$_{2}$ and Ni(NCS)$_{2}$, respectively, we
identify significantly stronger net antiferromagnetic interactions between the
earlier TM ions-a decrease in the Weiss constant, \theta, from 29 K for
Ni(NCS)$_{2}$ to -115 K for Mn(NCS)$_{2}$-a consequence of more diffuse 3d
orbitals, increased orbital overlap and increasing numbers of unpaired
$\it{t}$$_{2g}$ electrons. We elucidate the magnetic structures of these
materials: Mn(NCS)$_{2}$, Fe(NCS)$_{2}$ and Co(NCS)$_{2}$ order into the same
antiferromagnetic commensurate ground state, whilst Ni(NCS)$_{2}$ adopts a
ground state structure consisting of ferromagnetically ordered layers stacked
antiferromagnetically. We show that magnetic molecular frameworks with
significantly stronger net exchange interactions can be constructed by using
earlier TMs.
