There are rules for naming coordination complex, but people usually makes some mistakes even the rules are in front of them. So below are the most common mistakes people can make when naming coordination complex. After reading this, I believe you wll never make these mistakes agian. 

   Let's use CrCl₂(en)₂ as an example first. In this question, we can find the charge first. "en" means ethylenediammine, which has no charge. 2 Cl gives you 2- charge ( each Cl- gives you 1- charge). So since the whole coordination complex has a net charge of zero, Cr should have a 2+ charge.   (2+) (from Cr)  +  (2-) (from 2Cl^-) +0 (from en) = 0 

Step 2 is, specify the number and name of the Cl- ligands. Use prefixes we learnt long long time ago for naming the other organic molecuels such as 2 - di, 3- tri , 4- tetra, 5- penta, 6 - hexa, so on so forth. So we get dichloro here.   (Remember "mono" is not used here.)

Step 3, specify the number and name of the  ligands using prefixes such as 2 - bis, 3- tris, 4- terakis and so on. I know this sounds wierd, but we have no choice but to do this because the ligand  en (ethylenediamine) already contains the prefix "di" in it, using same set of prefixes will cause of confusion, and that is why scientists uses a different set of prefixes here. Now we get bis(ethylenediamine). Hey, notice that the prefix 'bis" is not in parentheses! The most common mistake is to put these prefixes into parentheses!

Step 4, Put a Roman numeral [ such as 1-(I), 2- (II), 3 - (III), 4 - (IV),etc]  in parentheses after the name of the metal to specify the oxidation state of the metal. Here we get chromium (II) because of the 2+ charge.

Step 5, just join all these parts together. From step 2 to 4, we get  dichloro  , the bis(ethylenediamine), then chromium (II). So we get dichlorobis(ethylenediamine)chromium (II).  Why dichloro goes first? Well, that is not because of the arrangement in formula, but because of alphabetical order. Notice that chloro (starts with C) should be before ethylenediamine (starts with E). By the way,we don't consider alphabetical order of prefixes when arranging these things. Notice that there should be any spaces in between the whole name. This is a whole, so just put them together!

Summerize all of the above, here are the most common mistakes you can make when naming coordination complex:

1. The ethylenediamine ligand is an exception. The prefixes used are bis, tris, and tetrakis instead of di, tri, and tetra.

2.The names of complex ligands such as ethylenediammine should be enclosed in parentheses, while their prefixes are not in parentheses.

3. Use Roman numeral for metal, don't use the prefixes like di, tri, tetra or bis, tris, tetrakis for metals.

4. No spaces between the whole name. But for [Ni(H₂O)₃(CO)]SO₄, it is called triaquacarbonylnickel(II) sulfate, there is a space between triaquacarbonylnickel(II) and sulfate because in the formula there is a parentheses sperating these two.