The Uniqueness of the $\Theta^+$ Pentaquark
  The existence of the $\Theta^+$ pentaquark requires a peculiar mechanism to explain its stability. Looking at quark clusters, notably diquark and triquark configurations, such a mechanism may be found in the color-magnetic interaction between quarks. It is possible to understand why the $\Theta^+$ is unique. Chiral dynamics, in particular the ease of pion emission, will render other members of the same flavor antidecuplet, such as the $\Xi^{--}$ very unstable. 
P.J. Mulders and P. Jimenez Delgado