Can a Neapolitan sixth chord be used as seventh and if so what is it called?

A Neapolitan sixth chord can be used as a seventh chord, in which case it is called a Neapolitan seventh chord. The Neapolitan seventh chord is a major seventh chord with a flatted second, which gives it a distinctive and somewhat melancholy sound. It is often used in classical music to create a sense of tension or drama.

Here is an example of a Neapolitan seventh chord:

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C - E♭ - G - B♭

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This chord could be used as the seventh chord in the key of C major, or as a substitute for the dominant seventh chord in the key of G minor.

Neapolitan seventh chords can also be used in jazz and other contemporary genres of music. They are often used to create a sense of tension or drama, or to add a touch of color to a chord progression.