The electronic configuration of Cr2+→1s22s22p63s23p63d4.
It has a d4 configuration. While acting as a reducing agent, it gets oxidized to Cr3+ (electronic configuration, d3). This d3 configuration can be written as t32g configuration, which is a more stable configuration.
Ni2+ is an oxidizing agent.
Ni2+→1s22s22p63s23p63d8.
It tends to accept two electrons and get reduced to achieve d10 configuration which is a more stable configuration.