Molecular Structure of Dicyclopentadienylnickel (C5H5)2Ni

The structure of gaseous nickelocene has been determined at 110°C by electron diffraction using two independent sets of data. The molecule has the familiar ferrocenelike sandwich shape, but it differs from ferrocene in having weaker metal–carbon bonds and greater amplitudes of vibration for all distances affected by motion of the cyclopentadienyl rings as units. The simplest interpretation of the large inter‐ring amplitudes leads to the conclusion that the rings are rotating much more freely than in ferrocene. There is some slight evidence that the Ni–C bonds may not all be equivalent, corresponding to a structure that is not strictly symmetric ($D_{\text{2h}}$ or $D_{\text{2d}}$ ), or alternatively that the molecular motions affecting the Ni–C bonds are unusual in a way which does not give rise to the expected essentially harmonic Ni–C amplitudes, either of which suggests the faint possibility of a Jahn–Teller effect. Based upon a symmetric sandwich conformation with freely rotating rings the principal distance ${\mathrm{(r}}_a)$ and mean amplitude ${\mathrm{(l}}_a)$ values are (values in parentheses are $\text{2σ}$ ): $r_{\mathrm{Ni–C}}\text{\hspace{0.17em}=\hspace{0.17em}2.196 Å (0.008)}$ , $r_{\mathrm{C–C}}\text{\hspace{0.17em}=\hspace{0.17em}1.430 Å (0.003)}$ , $r_{\mathrm{C–H}}\text{\hspace{0.17em}=\hspace{0.17em}1.083 Å (0.019)}$ , ∠C₅,H (angle between the plane of the C₅ ring and the C–H bond) = 0.28° (2.90), $l_{\mathrm{Ni–C}}{\text{\hspace{0.17em}=\hspace{0.17em}0.0842 Å (0.005}}_6)$ , $l_{\mathrm{C–C}}{\text{\hspace{0.17em}=\hspace{0.17em}0.0440 Å (0.003}}_1)$ , $l_{\mathrm{C–H}}{\text{\hspace{0.17em}=\hspace{0.17em}0.0786 Å (0.022}}_9)$ , $l_{\mathrm{Ni···H}}{\text{\hspace{0.17em}=\hspace{0.17em}0.1469 Å (0.034}}_6)$ .