Steric vs. electronic anomaly observed from iodomethane oxidative addition to tertiary phosphine modified rhodium(i) acetylacetonato complexes following progressive phenyl replacement by cyclohexyl [PR(3) = PPh(3), PPh(2)Cy, PPhCy(2) and PCy(3)].

Rhodium(i) acetylacetonato complexes of the formula [Rh(acac)(CO)(PR(3))] [acac = acetylacetonate, PR(3) = PPh(3) 1, PCyPh(2) 2, PCy(2)Ph 3, PCy(3) 4] were synthesized and the iodomethane oxidative addition to these complexes were studied. Spectroscopic and low temperature (100 K) single crystal X-ray crystallographic data of the rhodium complexes (1-4) indicate a systematic increase in both steric and electronic parameters of the phosphine ligands as phenyl groups on the tertiary phosphine are progressively replaced by cyclohexyl groups in the series. Second order rate constants for the alkyl formation in the oxidative addition of iodomethane in dichloromethane at 25 degrees C vary with approximately one order-of-magnitude from 6.98(6) x 10(-3) M(-1)s(-1) (PCyPh(2)) to 55(1) x 10(-3) M(-1) s(-1) (PCy(2)Ph 3) and do not follow the expected electronic pattern from to 1-4, which indicates a flexibility of the cyclohexyl group, significantly influencing the reactivity. Activation parameters for the reactions range from 35(3) to 44(1) kJ mol(-1) for DeltaH( not equal) and -140(5) to -154(9) J K(-1) mol(-1) for DeltaS( not equal), and are supporting evidence for an associative activation for the oxidative addition step.