X+
They are 1,2-dipolar species of the type RmX+ - Y-Rn.  If X is a saturated atom from the first row of the periodic table, then this type of compound is represented by a charge-separable form, but if X is an unsaturated atom doubly bonded to another first row element Z, the negative charge on Y may be stabilized by a pi conjugation.  Defined as compounds in which an anionic site Y- is attached directly to a heteroatom X+ carrying a formal positive charge these are, FTP, what compounds whose phosphonium variety is also known as the Wittig reagent?
They are 1,2-dipolar species of the type RmX+ - Y-Rn.  If X is a saturated atom from the first row of the periodic table, then this type of compound is represented by a charge-separable form, but if X is an unsaturated atom doubly bonded to another first row element Z, the negative charge on Y may be stabilized by a pi conjugation.  
ylides