This excitingly varied, Leipzig-centric program-the BSO's first "Leipzig Week in Boston"-celebrates Andris Nelsons and the BSO's compelling new collaboration with the venerable Leipzig Gewandhaus Orchestra by featuring three composers strongly associated with that city, plus a new work jointly commissioned by both ensembles from the accomplished American orchestral composer Sean Shepherd, a Tanglewood Music Center alumnus now based in New York City. The opener brings together three world-class virtuoso pianists for Bach's triple keyboard concerto, BWV 1063, possibly created for performances involving his two elder sons, W.F. and C.P.E Bach, at Zimmermann's coffeehouse in 1730s Leipzig. Closing the concert is the deeply Romantic Scottish Symphony of Felix Mendelssohn, who was music director of the Gewandhaus from 1835 to 1847. And it was Leipzig where Robert Schumann met his wife Clara and spent much of his early career; his two contrasting, rarely heard works for chorus and orchestra on this program date from the late 1840s.