Yes, METATHESIS is a Scrabble word!

Letter values in Scrabble US: M 3, E 1, T 1, A 1, T 1, H 4, E 1, S 1, I 1, S 1.

Is it a valid word?

METATHESIS is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

a linguistic process of transposition of sounds or syllables within a word or words within a sentence
a chemical reaction between two compounds in which parts of each are interchanged to form two new compounds (AB+CD=AD+CB)

Also: double decomposition, double decomposition reaction.