An unusual field-dependent slow magnetic relaxation and two distinct transitions were observed in a family of rare-earth\char21{}transition-metal complexes [Ln (bipy) $({\mathrm{H}}_{2}\mathrm{O}{)}_{4} M(\mathrm{CN}{)}_{6}] \ensuremath{\cdot}1.5$ (bipy) $\ensuremath{\cdot}4{\mathrm{H}}_{2}\mathrm{O}$ $(\mathrm{b}\mathrm{i}\mathrm{p}\mathrm{y}\phantom{\rule{0ex}{0ex}}={2,2}^{\ensuremath{'}}$-bipyridine; ${\mathrm{Ln}\mathrm{}=\mathrm{}\mathrm{Gd}}^{3+},{\mathrm{Y}}^{3+};$ $M={\mathrm{Fe}}^{3+},$ ${\mathrm{Co}}^{3+}).$ The magnetic relaxation, which is quite different from those in normal spin glasses and superparamagnets but qualitatively resembles those in the single-molecule magnet ${\mathrm{Mn}}_{12}\ensuremath{-}\mathrm{Ac}$ even if they possess different structures, might be attributed to the presence of frustration that is incrementally unveiled by the external magnetic field. The two distinct transitions in [GdFe] were presumed from dc and ac susceptibility as well as heat capacity measurements.