  A C$_3$-symmetric crystal-field potential in the Fe(II)Fe(III) bimetallic
oxalates splits the L=2 Fe(II) multiplet into two doublets and a singlet. In
compounds that exhibit magnetic compensation, one of the doublets was predicted
to lie lowest in energy and carry a non-quenched orbital angular momentum $\pm
\ld $, where $\ld $ exceeds a threshold value. In a range of $\ld $, a
Jahn-Teller (JT) distortion increases the energy splitting of the low-lying
doublet and breaks the C$_3$ symmetry of the bimetallic planes around the
ferrimagnetic transition temperature. At low temperatures, the JT distortion
disappears in compounds that display magnetic compensation due to the
competition with the spin-orbit coupling. A comparison with recent measurements
provides strong evidence for this re-entrant, low-temperature JT transition and
a prediction for the normal, high-temperature JT transition. The size of the JT
distortion is estimated using first-principles calculations, which suggest that
the long-range ordering of smaller, non-C$_3$-symmetric organic cations can
eliminate magnetic compensation.
