Molecular Structure and Bonding of InC5H5

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Abstract
The molecular structure of gaseous indium cyclopentadienyl was investigated by electron diffraction. Diffracted intensities revealed the molecule to be an ``open-faced half-sandwich'' with a symmmetry at least closely approximating C5v. The principal mean distances and vibrational amplitudes found were 2.621±0.005 Å and 0.077±0.007 Å for In–C and 1.427±0.007 Å and 0.040±0.009 Å for C–C. It was observed that the C–H bonds are pushed 4.5°±2° out of the plane of the carbon ring by the indium atom. The failure of the Born approximation significantly lowered the sensitivity of the analysis but did not prevent the deduction of reasonably good structural parameters. Metal—carbon bonds were 0.4 Å shorter than the sum of the ionic radii but both the In–C and C–C bond lengths agreed well with calculations based on Pauling's theory of covalent bonds of fractional bond order. Moreover, metal—ring overlap integrals were found to be of substantial magnitude. Therefore it is reasonable to conclude that the binding in the complex is essentially covalent, contrary to some previously published conjectures.