  The competition of the free spin state of a paramagnetic impurity on a
superconductor with its screened counterpart is characterized by the energy
scale of Kondo screening $k_BT_K$ compared to the superconducting pairing
energy $\Delta$. When the experimental temperature suppresses Kondo screening,
but preserves superconductivity, i.e., when $\Delta/k_B > T > T_K$, this
description fails. Here, we explore this temperature range in a set of
manganese phthalocyanine molecules decorated with ammonia (MnPc-NH$_3$) on
Pb(111). We show that these molecules suffice the required energy conditions by
exhibiting weak-coupling Kondo resonances. We correlate the energy of the
Yu-Shiba-Rusinov (YSR) bound states inside the superconducting energy gap with
the intensity of the Kondo resonance. We reveal that the bound state energy
follows the expectations for a classical spin on a superconductor. This finding
is important in view of many theoretical predictions using a classical spin
model, in particular for the description of Majorana bound states in magnetic
nanostructures on superconducting substrates.
