The bromine atom is found to be virtually inert to nucleophilies. Despite the  formal resemblance in the environment of the bromine atom in to that in, they  are found to differ in their rate of reaction under parallel conditions by a factor  of $\approx 10^{-23} : 1!$ This is because stabilisation of the carbocation in $(I)$ can occur by delocalisation of its charge through the $\pi$ -orbital systems  of the three benzene rings; whereas the extremely rigid structure of  $(II)$ will hold the cation's empty orbital (from loss of $Br^{\ominus}$ ) but at  right angles to these $\pi$-orbital systems, thus preventing such delocalisation.