  Using the data on the superconducting critical temperature ($T_{C}$) for a
number of metal hydrides, we found a rule that makes it possible to predict the
maximum $T_{C}$ based only on the information about the electronic structure of
metal atoms. Using this guiding principle, we explored the hydride systems for
which no reliable information existed, predicted new higher hydrides in the
K-H, Zr-H, Hf-H, Ti-H, Mg-H, Sr-H, Ba-H, Cs-H, and Rb-H systems at high
pressures, and calculated their $T_{C}$. Results of the study of actinides and
lanthanides show that they form highly symmetric superhydrides $XH_{7-9}$.
However, actinide hydrides do not exhibit high-temperature superconductivity
(except Th-H system) and might not be considered as promising materials for
experimental studies, as well as all $d^m$-elements with m > 4, including metal
hydrides of the noble elements. Designed neural network allowing the prediction
of $T_{C}$ of various hydrides shows good accuracy and was used to estimate
upper limit for $T_{C}$ of the materials with absence of the data. The
developed rule, based on regular behavior of the maximum achievable critical
temperature as a function of number of $d+f$ electrons, enables targeted
predictions about the existence of new high-$T_{C}$ superconductors.
