  A compound with very weakly interacting chains, MnCl$_3$(bpy), has attracted
a great deal of attention as a possible $S=2$ Haldane chain. However,
long-range magnetic order of the chains prevents the Haldane gap from
developing below 11.5~K. Based on a four-sublattice model, a description of the
antiferromagnetic resonance (AFMR) spectrum up to frequencies of 1.5~THz and
magnetic fields up to 50~T indicates that the interchain coupling is indeed
quite small but that the Dzaloshinskii-Moriya interaction produced by broken
inversion symmetry is substantial (0.12~meV). In addition, the
antiferromagnetic, nearest-neighbor interaction within each chain (3.3~meV) is
significantly stronger than previously reported. The excitation spectrum of
this $S=2$ compound is well-described by a $1/S$ expansion about the classical
limit.
