The Eighteen Electron Rule has not been used correctly, because there are 21 electrons. You have confused C₆H₅ (phenyl) with C₆H₆ (benzene). Phenyl forms 1 covalent bond a metal (¹) whilst benzene could form up to 6 bonds to the metal (⁶).
 

Pt              = 10e- Br             = 1e- (6-C6H6) = 6e- 2 x Pr3P    = 4e-                      21e-

Back to problems