  We present the first experimental realization of an $S=2$
ferromagnetic-antiferromagnetic (F-AF) alternating chain in a new Mn-verdazyl
complex [Mn(hfac)$_2$]$\cdot$($o$-Py-V)
[hfac=1,1,1,5,5,5-hexafluoroacetylacetonate;
$o$-Py-V=3-(2-pyridyl)-1,5-diphenylverdazyl]. Through the $ab$ $initio$
molecular orbital calculation, magnetization, and ESR measurements, this
compound is confirmed to form an $S=2$ F-AF alternating chain with Ising
anisotropy below about 100 K. Furthermore, we find an anomalous change in
magnetization at 1/4 of the saturation value, which is probably a manifestation
of the quantum nature of the system.
