  Metal-organic magnets (MOMs), modular magnetic materials where metal atoms
are connected by organic linkers, are promising candidates for next-generation
quantum technologies. MOMs readily form low-dimensional structures, and so are
ideal systems to realise physical examples of key quantum models, including the
Haldane phase, where a topological excitation gap occurs in integer-spin
antiferromagnetic (AFM) chains. Thus far the Haldane phase has only been
identified for $S=1$, with $S \geq 2$ still unrealised because the larger spin
imposes more stringent requirements on the magnetic interactions. Here, we
report the structure and magnetic properties of CrCl$_2$(pym) (pym=pyrimidine),
a new quasi-1D $S=2$ AFM MOM. We show, using X-ray and neutron diffraction,
bulk property measurements, density-functional theory calculations and
inelastic neutron spectroscopy (INS) that CrCl$_2$(pym) consists of AFM
CrCl$_2$ spin chains ($J_1=-1.13(4)\;$meV) which are weakly ferromagnetically
coupled through bridging pym ($J_2=0.10(2)\;$meV), with easy-axis anisotropy
($D=-0.15(3)\;$meV). We find that although small compared to $J_1$, these
additional interactions are sufficient to prevent observation of the Haldane
phase in this material. Nevertheless, the proximity to the Haldane phase
together with the modularity of MOMs suggests that layered Cr(II) MOMs are a
promising family to search for the elusive $S=2$ Haldane phase.
