How can one compare the $\ce{C-Cl}$ bond lengths in chloromethane ($\ce{CH3Cl}$) and chlorotrifluoromethane ($\ce{CF3Cl}$)?
Bent's rule is also consistent with Gillespie's VSEPR model, and may provide alternative rationalisation for effect of electronegativity. So it is restated as: 'more electronegative atom not only prefers to stay in the orbital having more $\mathrm{p}$ character but also can increase the $\mathrm{p}$ character in its attached orbital from the central atom depending on the circumstance.'
With increase in $\mathrm{p}$ character in an orbital, bond length will increase while with increase in $\mathrm{s}$ character in an orbital, bond length will decrease. For example, $d_\ce{(C-Cl)}$ in $\ce{CH3Cl}$ ($\pu{1.78 \unicode{xC5}}$) $\gt d_\ce{(C-Cl)}$ in $\ce{CF3Cl}$ ($\pu{1.75 \unicode{xC5}}$).
Source: Concise Inorganic Chemistry for JEE (by J.D. Lee and Sudarshan Guha)
I understand this alternative statement to Bent's rule. Can anybody explain how have the $\ce{C-Cl}$ bond lengths been compared here? I don't get this. I understand that fluorine is more electronegative than hydrogen and might induce small positive charge on the carbon atom. Also, the $\ce{C-F}$ bond will have more $\mathrm{p}$ character than the $\ce{C-H}$ bond. How do I proceed further to reach the conclusion?