Which among the following is incorrect statement?

To determine which of the given statements is incorrect, let us analyze the electromeric effect (E-effect) and evaluate each option step-by-step.

1. Understanding the Electromeric Effect:
The electromeric effect is a temporary organic reaction mechanism. It involves the complete transfer of a shared pair of $\pi$-electrons to one of the atoms joined by a multiple bond (double or triple bond) in the presence of an attacking reagent. Since it only occurs when an attacking reagent is present and vanishes as soon as the reagent is removed, it is a temporary effect. Thus, the statement "The electromeric effect is, temporary effect" is correct.
Similarly, the statement "The organic compound shows electromeric effect in the presence of the reagent only" is also correct.

2. Electromeric Effect vs. Inductive Effect:
When both inductive and electromeric effects operate in opposite directions in a molecule, the electromeric effect generally dominates over the inductive effect because it involves the complete transfer of electrons rather than just a partial displacement. Thus, the statement "Electromeric effect dominates over inductive effect" is correct.

3. Types of Electromeric Effect:
The electromeric effect is classified into two types based on the direction of electron transfer relative to the attacking reagent:
• Positive Electromeric Effect (+E effect): This occurs when the $\pi$-electrons of the multiple bond are transferred to the atom to which the attacking reagent gets attached. For example, when a hydrogen ion ($H^{+}$) attacks an alkene, the $\pi$-electrons shift towards the carbon atom where the $H^{+}$ adds:
$>C=C<+H^{+}\to >C^{+}-C\left(H\right)<$
Since the electrons shift towards the carbon atom that binds with $H^{+}$, $H^{+}$ shows a positive electromeric effect (+E effect), not a negative one.
• Negative Electromeric Effect (-E effect): This occurs when the $\pi$-electrons are transferred to the atom other than the one to which the attacking reagent gets attached (e.g., the attack of cyanide ion $C{N}^{-}$ on a carbonyl group).

Therefore, the statement "Hydrogen ion (H⁺) shows negative electromeric effect" is incorrect.