  We use a constituent model to analyze the stability of pentaquark $\bar Q
qqqq$ configurations with a heavy antiquark $\bar c$ or $\bar b$, and four
light quarks $uuds$, $ddsu$ or $ssud$. The interplay between chromoelectric and
chromomagnetic effects is not favorable, and, as a consequence, no bound state
is found below the lowest dissociation threshold.
