The frequency of MO stretching vibration frequency increases

The frequency of M-O stretching vibration frequency increases for the octahedral aquo ions in the following manner Ca+2 < Mn+2 < Ni+2 Compare this trend with acidity and rationalize.

Solution

The frequency of the symmetrical M-O stretching vibration of the octahedral aqua ions [M(OH₂)₆]²⁺increases along the series Ca²⁺ < Mn²⁺ < Ni²⁺

The same trend is followed by acidity of compounds of these ions

Explanation:

The trend in symmetrical M–O stretching vibration frequencies relates very well with the acidities of these aqua acids. The frequencies increase in the order Ca²⁺ < Mn²⁺ < Ni²⁺, the same order that the M–O bond strength increases. This is a reasonable observation because in the same order the radii of M²⁺ cations are decreasing and charge density is increasing. With increasing charge density the attraction between M²⁺ cation and partially negative O atom in H₂O is increasing. For the same reason, the acidity of these aqua acids is increasing showing that two trends correlate very closely.