[lord12]

I can't quite figure out the number of d electrons in [Au(CO)4]3-. I looked it up and there are 8 d electrons. But I want to onown the reasoning.

[Yggdrasil]

If you know the charge of the ligands and the charge of the complex, you can figure out the charge of the metal ion.  From the charge of the metal ion and its atomic number, you can figure out how many electons it has.

[swati]

In this case carbonyl that is CO is a neutral ligand, so calculate the charge on ligand

[mdlhvn]

I can't quite figure out the number of d electrons in [Au(CO)4]3-. I looked it up and there are 8 d electrons. But I want to onown the reasoning.

Maybe you confuse the fomula. It must be [Au(CO)4]+ or [Au(CO)4]3+. I think 3+ is more suitable. Because CO is the neutral ligand, the charge of the complex is equal to that of metal ion. The electron configuration of Au is 5d¹⁰6s¹, so Au3+ must have 8 d electrons