  The magnetic ground state of two isostructural coordination polymers (i) the
quasi two-dimensional S = 1/2 square-lattice antiferromagnet
[Cu(HF$_{2}$)(pyrazine)$_{2}$]SbF$_{6}$; and (ii) a new compound
[Co(HF$_{2}$)(pyrazine)$_{2}$]SbF$_{6}$, were examined with neutron powder
diffraction measurements. We find the ordered moments of the Heisenberg S = 1/2
Cu(II) ions in [Cu(HF$_{2}$)(pyrazine)$_{2}$]SbF$_{6}$ are 0.6(1)$\mu_{B}$,
whilst the ordered moments for the Co(II) ions in
[Co(HF$_{2}$)(pyrazine)$_{2}$]SbF$_{6}$ are 3.02(6)$\mu_{B}$. For Cu(II), this
reduced moment indicates the presence of quantum fluctuations below the
ordering temperature. We show from heat capacity and electron spin resonance
measurements, that due to the crystal electric field splitting of the S = 3/2
Co(II) ions in [Co(HF$_{2}$)(pyrazine)$_{2}$]SbF$_{6}$, this isostructual
polymer also behaves as an effective spin-half magnet at low temperatures. The
Co moments in [Co(HF$_{2}$)(pyrazine)$_{2}$]SbF$_{6}$ show strong easy-axis
anisotropy, neutron diffraction data which do not support the presence of
quantum fluctuations in the ground state and heat capacity data which are
consistent with 2D or close to 3D spatial exchange anisotropy.
