Name the complex ion [cr(cn)6]3−. the oxidation number of chromium is +3. enter the name.

The given complex Ion is,

                                          [Co(CN)₃]³⁻

In complex compounds the Positive part is named first, followed by name of Negative part. In given case there is no Positive part. So, we will name the Negative part.

In Sphere the Ligands are named first. In this case there are 6 CN⁻ ligands. As CN⁻ (cyanide) contains a negative charge so the -ide is replaced by -o. i.e. Cyanide → Cyano. Also, Hexa is placed before Cyano as there are 6 CN⁻ Ligands.
So,
                                           Hexacyano

After that Chromium metal is named. As there is a negative charge on the sphere, so the -ium in Chromium is replaced by -ate.
i.e. Chromium → Chromate. And the oxidation state of Cr is written in Roman. Hence,the name is as,
          
                                Hexacyanochromate (III) Ion