Isoelectronic species are atoms and ions that have the same number of electrons.
A pair of species having the same number of electrons combining all the atoms of a particular species is called an isoelectronic pair.
Because of different nuclear charges, the radii of isoelectronic species would differ.
In CN- species, the number of electrons would be that is 6 +7 +1 = 14
.In CO species, the number of electrons would be that is 6 + 8 = 14