How many of the following species are paramagnetic: O2^2-, B2, NO, [Ni(CN)4]^2-, [NiCl4]^2-, [Fe(CN)6]^4-, [CoF6]^3-, C2? Fe 26, Co 27, Ni 28…

Count unpaired electrons in each. $\mathrm{O_2^{2-}}$ (peroxide, 18 electrons): all $\pi^*$ orbitals filled, bond order 1, diamagnetic. $\mathrm{B_2}$ (10 electrons): the last two electrons occupy degenerate $\pi_{2p_x}$ and $\pi_{2p_y}$ singly, paramagnetic. $\mathrm{NO}$ (15 electrons) has one electron in $\pi^*$, paramagnetic. $\mathrm{[Ni(CN)_4]^{2-}}$: $\mathrm{Ni^{2+}}$ $d^8$, square planar strong field, all paired, diamagnetic. $\mathrm{[NiCl_4]^{2-}}$: $d^8$ tetrahedral, two unpaired electrons, paramagnetic. $\mathrm{[Fe(CN)_6]^{4-}}$: $\mathrm{Fe^{2+}}$ $d^6$ low spin $t_{2g}^6$, diamagnetic. $\mathrm{[CoF_6]^{3-}}$: $\mathrm{Co^{3+}}$ $d^6$, weak-field $\mathrm{F^-}$, high spin with four unpaired electrons, paramagnetic. $\mathrm{C_2}$ (12 electrons) has both electrons paired in $\pi_{2p}$ orbitals, diamagnetic. Paramagnetic species: $\mathrm{B_2}$, NO, $\mathrm{[NiCl_4]^{2-}}$, $\mathrm{[CoF_6]^{3-}}$, so the answer is 4.