Why oxides with the element in higher oxidation state are more acidic

We see this trend of acidic-basic property of oxides in most cases:
$\ce{Mn2O7}$ is acidic, $\ce{MnO}$ is basic, the intermediate oxides are progressively less acidic and more basic, as the oxidation state of $\ce{Mn}$ decreases.


What is the reason of this general trend in almost all cases?


I have seen many explanations of this, most of them involving the proton releasing capacity of $\ce{E-O-H}$ bond, in the hydrolysed oxide. But most oxides of transition metals do not hydrolyze in water. So, I am seeking an alternative explanation without having to involve the hydrolysis of oxide. Also, most explanations on the internet lack any reference, so I prefer a reference.




It does not appear to be always true. According to [Wikipedia](https://en.m.wikipedia.org/wiki/File:Copper_in_water_pourbiax_diagram.png), copper(I) becomes acidic, forming an anionic hydroxide complex, at about 1.5 pH unit *lower* than copper (II). The higher oxidation state of copper qualifies as less acidic.


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A simple answer for this would be to say that $\ce{Mn2O7}$ has +7 oxidation state. Thus it is electron deficient and behaves as a lewis acid.
$\ce{MnO}$ has +2 oxidation state and it is comparatively electron rich. Thus it behaves as a lewis base. So compounds in the middle are amphoteric.


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In the bonds $M^{z+}-O-H$, where $M$ can be $Mn$ or $Cr$, the atom $M$ is positively charged. When this charge $z$ is large, $M$ repells the $H$ atom stronger than if z is small. So the molecule containing these bonds is a stronger acid. For example, $Z$ = $6$ in $H\_2CrO\_4$ and $Z = 7$ in $HMnO\_4$. They both contain at least one $M-O-H$ bond. As $Z$ is high in these molecules, they are strong acids. Molecules containing $H, O$ and the same atoms $Mn$ and $Cr$ at a lower oxidation state exist. But they are not acidic in water. Examples : $Mn^{2+}$ makes $Mn(OH)\_2$ which is insoluble in water and not acidic. $Cr^{3+}$ and $Cr^{2+}$ both produces $Cr(OH)\_3$ and $Cr(OH)\_2$ which are also insoluble and not acidic. 


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