Trends In Stability of Higher Oxidation States

TiX4 ( tetrahalides ) , VF5  and CrF6 are having highest oxidation numbers .

Mn , for its +7 state , is not represented in simple halides but MnO3F is known .  

You will find beyond Mn has trihalide except FeX3 and CoF3 . 

Due to higher lattice energy or higher bond enthalpy , fluorine can stabilise the highest oxidation states . 

Trends in the M2+ / M Standard Electrode Potentials

In these trends, you can observe the unique behaviour of copper. Unlike many other metals, the standard electrode potential of copper is positive. This positive value explains why copper does not liberate hydrogen gas when it dissolves in ordinary acids. Other transition metals with more negative values of electrode potential behave differently.

Because of this property, copper does not react with dilute acids like HCl or H₂SO₄ to release hydrogen gas. Only strong oxidizing acids such as nitric acid (HNO₃) and concentrated sulphuric acid (H₂SO₄) are capable of reacting with copper. In these cases, copper is oxidized, but hydrogen gas is not produced; instead, gases like NO, NO₂, or SO₂ are formed.

The reason behind this unusual behaviour lies in copper’s hydration enthalpy. The hydration enthalpy of Cu²⁺ ions is not sufficient to balance the high ionization energy required to transform a copper atom into a Cu²⁺ ion. As a result, copper remains less reactive compared to metals like manganese, zinc, and nickel.

On the other hand, metals like Mn, Zn, and Ni possess more negative standard electrode potentials. This indicates they are stronger reducing agents and can easily displace hydrogen from dilute acids, readily producing hydrogen gas.