Among $$\text{Ni}^{4+}$$, $$\text{Cu}^{2+}$$, $$\text{Mn}^{4+}$$, $$\text{Co}^{3+}$$, $$\text{Cr}^+$$ the species having same number of unpaired electrons ____________

PLACEHOLDER ANSWER - the answer is a pair of ions. The configurations are $$\text{Ni}^{4+}$$ $$3d^6$$ (4 unpaired), $$\text{Cu}^{2+}$$ $$3d^9$$ (1), $$\text{Mn}^{4+}$$ $$3d^3$$ (3), $$\text{Co}^{3+}$$ $$3d^6$$ (4) and $$\text{Cr}^+$$ $$3d^5$$ (5). So $$\text{Ni}^{4+}$$ and $$\text{Co}^{3+}$$ have the same number, 4 unpaired electrons each.