Synthesis, Structures, Stabilities, and Reactions of Cationic Olefin Complexes of Palladium(II) Containing the η5-Cyclopentadienyl Ligand

Article abstract of DOI:10.1021/ja00543a016

Syntheses of various cationic olefin complexes of palladium(II), 5-C5H5)(PR3)(olefin)>X (R = Ph, Et, n-Bu; X = ClO4, BF4), 4, a class of compounds much more stable than hitherto known, are reported. 4 exhibited (1)H and (13)C NMR spectral aspects remarkably well-defined for an olefin-palladium complex, providing means of studying configurations and relative stabilities in some detail.In 4 containing substituted styrenes, (13)C NMR shifts of the olefin carbons correlate with the Hammet ?⁺ parameters, while stabilities of the complexes correlate better with ? than ?⁺.A possible significance of ion pair formation in determining stability trends has been suggested.Olefin ligands rotate more rapidly about the palladium-olefin axis than the palladium-olefin axis.This result, as well as a different substituent dependency of the stability in series of substituted styrene complexes of palladium(II) and platinum(II), is explained in terms of less effective ? back-bonding from palladium to olefin than from platinum.The ethylene complex of type 4 reacts with some nucleophiles to give alkyl complexes, Pd(η⁵-C5H5)(PPh3)(CH2CH2Y) , a class of compounds again remarkably stable for a substituted ethylpalladium(II) complex. (1)H NMR spectra of 9 and 10 prepared from the cis- and trans-ethylene-d2 complexes indicated the trans addition of Pd and Y to ethylene. 9 and 10 undergo thermolysis via β-hydrogen elimination which is suggested to proceed through predissociation of PPh3.The role of the η⁵-C5H5 ligand in raising stabilities of olefin and alkyl complexes of palladium(II) has been discussed in the light of the data.