coordination chemistry!! please help!!?

Question

what do the titles
d4, d5, d8, etc stand for and how do you find them??

for example, in my txt book it says that
Cr(H2O)6 (3+) is d5

Thank you!

Answer 1

Look at the Periodic Table. Co has to be 4s2 3d7. In Co(3+), the 4s2 electrons are gone and one 3d electron too. So Co(3+) has to be 3d6.

Answer 2

It is an abbreviation of the electron configuration: d5 means your atom (or ion) has 5 electrons in d orbitals, or an electronic configuration [ ] ns° (n-1)d5.
For Cr3+, however, the electronic configuration is [Ar] 4s° 3d3. (so it is a d3 ion)
This must be a typo in your textbook.