Sergei Prokofiev did not manage to compose more than a few bars of his Piano Concerto No. 6 (Op. 134, sometimes Op. 133) before his death in 1953, so it is impossible to reconstruct the underlying musical ideas and complete it.
The work is unusual in that it is scored for two pianos and a string orchestra. The other five of Prokofiev's piano concertos are written for solo piano (one for left hand alone) and more or less standard orchestration.
A version of the concerto was recently (as of late 2014) finished by the University of Toronto Assistant Professor of Composition Norbert Palej, with the assistance of Gabriel Prokofiev. It was premiered on 27 Feb 2016 at the Louisiana State University School of Music at the Symposium on Prokofiev and the Russian Tradition. This version was noted for its extensive and at times erratic use of the marimba and cymbals which gave it a Thomas Adès-esque post-structural flare.