How to calculate 18 electron rules in organometallic chemistry?i'm not understand the charge.

Question

calculation

Answer 1

Essentially you remove ligands with their pairs of electrons (or as closed shell molecules and ions).  So an ammonia ligand becomes NH3 which is neutral.  A chloro ligand becomes chloride which is -1.  A hydrogen ligand (hydride) becomes :H(-).  Cyclopentadienyl ligand becomes the cyclopentadienyl anion because that is aromatic.  

The metal takes on the charge necessary to balance all those.

So if you have CpMo(CO)3H, for example, cyclopentadienyl is (-1), the three CO ligands are neutral molecules so they're 0, and the hydride is -1 also, thus your Mo needs to be +2.  Now you count d electrons for Mo.  Mo is in group 6, so Mo(+2) is d4.  4 d electrons plus 6 electrons from the Cp + 2 from each carbonyl + 2 from the hydride gives you a total of 18.