13C and 19F NMR substituent chemical shifts (SCS) of some bridgehead-substituted phenyl- and fluorophenylbicyclo [2.2.2] octyltricarbonylchromium(O) derivatives: nature of aryl 19F NMR polar field effects in the Cr(CO)3-complexed benzene ring system

A number of Cr(CO)₃ complexes of bridgehead-substituted phenylbicyclo[2.2.2]octanes and (m- and p-)fluoropheylbicyclo[2.2.2]octanes have been synthesized and their ¹³C and ¹⁹F NMR spectra have been recorded, respectively. The substituent chemical shifts (SCS) of these stereochemically well-defined model systems permit an unambiguous evaluation of polar factors governing ¹³C and ¹⁹F SCS in arene-Cr(CO)₃ complexes. The dual nature of ¹⁹F NMR polar field effects is reaffirmed and the coefficient (A) of the Buckingham equation for linear electric field effects on C(sp²)F bonds in fluoroarene-Cr(CO)₃ complexes has been calculated. A re-examination and re-interpretation of the ¹⁹F chemical shifts of m- and p-substituted fluorophenyltricarbonylchromium derivatives is also reported. New substituent parameters (σ_I and σ^o_R) for C₆H₅ · Cr(CO)₃ as a substituent in the neutral ground state arepresented.