why is [Co(NH3)6]3+ d6?

Answer 1

the ion is hexamino Cobalt(III) cation

Because Cobalt is 4s2 3d7( Electronic configuration)

In this complex Co loses 3 electrons ......so the +3 charge.
's' electrons easiest to lose for transition elements.....2 electrons lost in this way!
the third electron is lost from the 'd' orbital so 3d7 becomes 3d6

Thus [Co(NH3)6]3+ is d6 for Co