Question

Is `"CoCl"_4^(2-)` paramagnetic or diamagnetic?

Answer 1

Cobalt in this case has an oxidation state of `+2` to add with the four `"Cl"^(-)` ligand charges and give an overall charge of `-2`. With a `+2` oxidation state, `"Co"` therefore is a `d^7` metal.

A four-coordinate complex with four `"Cl"^(-)` ligands (which are weak-field) generally is said, under crystal field theory, to have a small d-orbital splitting energy, making it high spin, since the `"Cl"^(-)`, treated as point charges, repel the metal `d` orbitals fairly little.

That makes the geometry easily tetrahedral (also favored since the metal is small). Thus, a `d^7` metal in a high spin four-coordinate complex would have a configuration of:

`ul(uarr color(white)(darr))" "ul(uarr color(white)(darr))" "ul(uarr color(white)(darr))" "(t_2)`
`3d_(xy)color(white)(.....)3d_(xz)color(white)(.....)3d_(yz)`

`" "ul(uarr darr)" "ul(uarr darr)" "" "" "(e)`
`" "3d_(z^2)color(white)(.....)3d_(x^2-y^2)`

Clearly, there are unpaired electrons, which makes the complex paramagnetic.