Pentacoordinate carbon atoms are theoretically predicted
here in a ferrocene dication derivative in both staggered-
[Fe(Si2C5H2)2]2+(1; C2h) and eclipsed-[Fe(Si2C5H2)2]2+(2; C2v) forms for the first time. The barrier between these two ranges from -38.11 to 4.90 kJ/mol at different levels. The planar tetracoordinate carbon atom in the ligand Si2C5H2 becomes a hypervalent pentacoordinate carbon upon complexation.