Why are transition-metal oxides with low oxidation numbers usually basic?

Transition-metal oxides with low oxidation numbers (typically +1, +2, and +3) are generally basic because they are ionic compounds consisting of distinct metal cations and oxide anions.1

In these lower oxidation states, the metal-oxygen bonds are ionic in character, which allows the oxides to react readily with aqueous acids to form salts and water. As the oxidation number of the metal increases, the bonding character shifts from ionic to covalent. Consequently, the acid-base behavior of these compounds changes: oxides with higher oxidation numbers (+5, +6, and +7) become increasingly acidic and tend to react with bases instead of acids. Oxides with an intermediate oxidation state of +4 are typically amphoteric, meaning they can behave as either an acid or a base depending on the environment.12