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

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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( sp 2)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 C 6H 5 · Cr(CO) 3 as a substituent in the neutral ground state arepresented.