ChemInform Abstract: Preparation und Characterization of the First Triscyclopentadienyl Lanthanoid Complexes Containing Two Aliphatic Nitrile Ligands: Crystal und Molecular Structures of the Isomorphous Compounds trans‐Bis(acetonitrile)tris(η5‐cyclopentadienyl)‐ Lanthanoid(III) (Ln: La, Ce, Pr). A Successful Confirmation of the ′Solid Angle Sum Rule′.