Sibelius’s No. 5 out of his Six Impromptus (Op. 5) is a petite B minor Vivace written in 1893. It’s only three and a half minutes, sounds mysterious, and somewhat very cool.
Sibelius was never really a piano composer, and you can hear him thinking in strings. He seems to have agreed. Within months he arranged this impromptu for string orchestra, later pairing it with No. 6, and the arrangement premiered in February 1894.
Leif Ove Andsnes plays it here as an encore in Milan: