  We review the properties of Ni-based superconductors which contain Ni2X2
(X=As, P, Bi, Si, Ge, B) planes, a common structural element found also in the
recently discovered FeAs superconductors. Strong evidence for the fully gapped
nature of the superconducting state has come from field dependent thermal
conductivity results on BaNi2As2. Coupled with the lack of magnetism, the
majority of evidence suggests that the Ni-based compounds are conventional
electron-phonon mediated superconductors. However, the increase in Tc in
LaNiAsO with doping is anomalous, and mimics the behavior in LaFeAsO.
Furthermore, comparisons of the properties of Ni- and Fe-based systems show
many similarities, particularly with regards to structure-property
relationships. This suggests a deeper connection between the physics of the
FeAs superconductors and the related Ni-based systems which deserves further
investigation.
