  We report on the low-temperature magnetic susceptibilities and specific heats
of the isostructural spin-ladder molecular complexes L$_{2}$[M(opba)]$_{3\cdot
x$DMSO$\cdot y$H$_{2}$O, hereafter abbreviated with L$_{2}$M$_{3}$ (where L =
La, Gd, Tb, Dy, Ho and M = Cu, Zn). The results show that the Cu containing
complexes (with the exception of La$_{2}$Cu$_{3}$) undergo long range magnetic
order at temperatures below 2 K, and that for Gd$_{2}$Cu$_{3}$ this ordering is
ferromagnetic, whereas for Tb$_{2}$Cu$_{3}$ and Dy$_{2}$Cu$_{3}$ it is probably
antiferromagnetic. The susceptibilities and specific heats of Tb$_{2}$Cu$_{3}$
and Dy$_{2}$Cu$_{3}$ above $T_{C}$ have been explained by means of a model
taking into account nearest as well as next-nearest neighbor magnetic
interactions. We show that the intraladder L--Cu interaction is the predominant
one and that it is ferromagnetic for L = Gd, Tb and Dy. For the cases of Tb, Dy
and Ho containing complexes, strong crystal field effects on the magnetic and
thermal properties have to be taken into account. The magnetic coupling between
the (ferromagnetic) ladders is found to be very weak and is probably of dipolar
origin.
