  We report the first detection of the metal-bearing molecules sodium sulfide
(NaS) and magnesium sulfide (MgS) and the tentative detection of calcium
monoxide (CaO) in the interstellar medium (ISM) towards the Galactic Center
molecular cloud G+0.693-0.027. The derived column densities are (5.0+-1.1) x
$10^{10}$ cm$^{-2}$, (6.0+-0.6) x $10^{10}$ cm$^{-2}$, and (2.0+-0.5) x
$10^{10}$ cm$^{-2}$, respectively. This translates into fractional abundances
with respect to H$_2$ of (3.7+-1.0) x $10^{-13}$, (4.4+-0.8) x $10^{-13}$, and
(1.5+-0.4) x $10^{-13}$, respectively. We have also searched for other Na-, Mg-
and Ca-bearing species towards this source but none of them have been detected
and thus we provide upper limits for their abundances. We discuss the possible
chemical routes involved in the formation of these molecules containing metals
under interstellar conditions. Finally, we compare the ratio between
sulfur-bearing and oxygen-bearing molecules with and without metals, finding
that metal-bearing sulfur molecules are much more abundant than metal-bearing
oxygen ones, in contrast with the general trend found in the ratios between
other non metal- oxygen- and sulfur-bearing molecules. This further strengthen
the idea that sulfur may be little depleted in G+0.693-0.027 as a result of the
low velocity shocks present in this source sputtering large amounts of material
from dust grains.
