the complex ion which has no d electrons in the central metal atom is

1. [Mno4]-
2. [co(NH3)6]3+
3. [Fe(CN)6]3-
4. [Cr(H2O)6]3+. how to solve this

in option A oxd state of Mn is +7
so there will be no d electrons in the central atom 

MnO₄^- is the correct answer as oxidation no. of Mn is +7 and electrons are not present in its d-orbital.

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[MnO₄]^-  AS ITHS OXIDATION STATE IS +7 AND 5 ELECTRONS IN D ORBITAL AND 2 ELECTRONS IN S ORBITAL....A TOTAL OF 7