  We investigated the magnetic behavior of metal hydrides FeH$_{x}$, CoH$_{x}$
and NiH$_{x}$ for several concentrations of hydrogen ($x$) by using Density
Functional Theory calculations. Several structural phases of the metallic host:
bcc ($\alpha$), fcc ($\gamma$), hcp ($\varepsilon$), dhcp ($\varepsilon'$),
tetragonal structure for FeH$_{x}$ and $\varepsilon$-$\gamma$ phases for
CoH$_{x}$, were studied. We found that for CoH$_{x}$ and NiH$_{x}$ the magnetic
moment ($m$) decreases regardless the concentration $x$. However, for FeH$_{x}$
systems, $m$ increases or decreases depending on the variation in $x$. In order
to find a general trend for these changes of $m$ in magnetic metal hydrides, we
compare our results with the Slater-Pauling curve for ferromagnetic metallic
binary alloys. It is found that the $m$ of metal hydrides made of Fe, Co and Ni
fits the shape of the Slater-Pauling curve as a function of $x$. Our results
indicate that there are two main effects that determine the $m$ value due to
hydrogenation: an increase of volume causes $m$ to increase, and the addition
of an extra electron to the metal always causes it to decrease. We discuss
these behaviors in detail.
