Wikipedia
Y-aromaticity
In organic chemistry, the term Y-aromaticity is used to describe a Y-shaped, planar (flat) molecule with resonance bonds that exhibits more stability than other geometric or connective arrangements with the same set of atoms.
It is developed to explain the extraordinary stability of guanidinium cation and high basicity of the Guanidine. Guanidinium is not a ring molecule, and is cross-conjugated rather than a linear π system, but is reported to have its six π-electrons delocalized over the whole molecule. The concept is controversial and some authors emphasize different effects.
Trimethylenemethane Dication has extraordinary stability than the Butadienyl Dication.