Cyclopentadienyl complexes with transition metal – main group metal bonds. VII. 207Pb and 95Mo nuclear magnetic resonance studies on molybdenum and tungsten complexes [(η5-C5H5)(CO)3M]nPbR4−n (n = 1, R = Me, Et, Ph; n = 2, R = Ph)

Nuclear magnetic resonance measurements of metallic nuclei (δ 207Pb, δ 95Mo, and 1J(W–Pb)) have been carried out on dimetallic Cp(CO)3MPbR3 (M = Mo, W; R = Me, Et, Ph) and trimetallic [Cp(CO)3M]2PbPh2 (M = Mo, W) complexes. 207Pb chemical shifts increase in the sequence PbEt3 < PbMe3 < PbPh3 and are observed at higher fields in the tungsten compounds than in the molybdenum ones. It is suggested that both paramagnetic and diamagnetic contributions determine the overall shielding of lead, and that the higher field resonances of 207Pb in the case of W–Pb compounds are due to an efficacious operation of the Z/r dependent diamagnetic term (higher atomic number Z of tungsten and roughly the same Mo–Pb and W–Pb distances resulting from the 4f level induced lanthanide contraction). 95Mo chemical shifts (−1838 to −2007 ppm) lie in the region typical of the molybdenum – main group metal bonded complexes but at lower fields than in the corresponding Mo–Sn compounds. It is concluded from this observation that the Z/r dependent diamagnetic contribution does not operate efficaciously for 95Mo shieldings, which are hence primarily dominated by the paramagnetic term. The nature of metal–metal interactions is discussed.