Bis(methylcyclopentadienyl) derivatives of germanium(II), tin(II), and lead(II): proton and carbon-13 NMR properties and comparison with those for analogs formed by univalent metals (lithium, potassium, and thallium)

Bis(methylcyclopentadienyl)germanium(II) has been isolated and found to be moderately stable toward polymerization.Comparison of and l3C NMR parameters with those for homoleptic tin and lead compounds indicates that in solution all three adopt a skew sandwich structure, first confirmed for Pb(C5H5)2 by vapor-phase electron diffraction.Carbon-13 chemical shifts for lithium, potassium, and thallium(I) derivatives of methylcyclopentadiene have been measured and relationships with those for the group 4 compounds are discussed in terms of bonding and electronegativity effects.In all cases a marked low-field displacement away from corresponding shifts for transition-metal complexes has been found.The tin-119 NMR spectrum of Sn(C5H4CH3)2 measured by the 'Hj119Sn} INDOR technique consists of a nonet, confirming equivalence at -40 °C between the two methylcyclopentadienyl rings, centered at more than 2100 ppm upfield from 119Sn shifts for Sn(IV) analogues.