Minor-key Mozart constitutes the thematic glue for this week’s Boston Symphony Orchestra program. The bill opens with his Piano Concerto No. 24 (K.491), one of two Mozart piano concertos in a minor key. (The BSO programmed the other one, No. 20, just last week.) It concludes with the darkest of all Mozart compositions, the Requiem, a piece that’s been haunted by ghost stories ever since its completion — or rather, its incompletion.

Written and premiered in 1786, K.491 is unusual in a number of other ways. It calls for oboes and clarinets, plus horns, trumpets, and timpani — the largest orchestra Mozart gave a piano concerto. Its first movement, like those of Nos. 11 and 14, is in 3/4. It begins and ends in C minor, the only Mozart piano concerto to finish in a minor key. The Allegro’s foreboding chromatic initial theme seems to anticipate Don Giovanni, which premiered the following year, and when the piano enters, after two minutes, it doesn’t restate that theme but rather reflects sadly on it, as if giving voice to one of Giovanni’s conquests.

I was taken aback by Nelsons’s less than forceful opening. It was scrupulously sculpted, but mellow enough to have come out of The Marriage of Figaro rather than Don Giovanni. All doubts were allayed, however, when his 71-year-old soloist, Romanian pianist Radu Lupu, entered. With weighted, thoughtful phrasing, Lupu created a long arc out of the Allegro; like Menahem Pressler, who played No. 27 with the BSO last November, he knew exactly where he was going and was in no hurry to get there. His immaculate passagework never called attention to itself, instead nestling into the orchestral framework. When he had to hop around in the bass, he did so without thumping. His cadenza, too, was an unostentatious exploration of ideas; there was no virtuosity for virtuosity’s sake.

The E-flat Larghetto is a study in simple beauty, and Lupu gave the songlike main theme a beguiling innocence, as if he were thinking of Zerlina. Here the piano is set mostly against the winds, which in the C-minor and A-flat middle sections chime with suppressed sorrow. Again like Pressler, Lupu was so unaffected, he made performing Mozart seem like child’s play.

The Allegretto finale, back in C minor, actually was child’s play, since Lupu and Nelsons turned it into a delightful toy march. The movement is structured as a theme with eight variations, and here each variation had its own character. The third soared, Schumann-like, with passionate support from the orchestra. The winds maintained a steady tread in the fourth; Lupu brought starlight to the fifth. Everything danced as well as marched, and that set up the final variation, where Mozart changes the time signature from 2/2 to 6/8.

This thought-provoking performance of No. 24 came from a soloist and conductor abiding on the same page. It was less dark and dramatic than some interpretations, leisurely but never limp. Lupu was so quiet in his chair, so bereft of physical flourishes, at times he seemed invisible. His music spoke for itself. He didn’t get anywhere near the ovation Mitsuko Uchida received for No. 20 last Thursday, but I enjoyed his Mozart a great deal more.    

As is well known by now, Mozart did not live to finish the Requiem. And there’s no want of titillating stories surrounding it. We’re told the commission for the piece was delivered by a mysterious messenger, perhaps Death himself. Mozart inferred from this visit that he was being commissioned to compose his own Requiem Mass. He didn’t finish it because he was poisoned by his arch-rival Antonio Salieri. (That hypothesis is developed in Peter Shaffer’s 1979 play Amadeus, and the subsequent 1984 film adaptation.) Or he was poisoned by his associate Franz Xaver Süssmayr because Franz Xaver was having an affair with Mozart’s wife, Constanze.

What we know of the truth is more mundane. Mozart did receive, in July of 1791, a commission to compose a Requiem Mass. The messenger, impersonating Death or otherwise, was probably Johann Nepomuk Sortschan, a law clerk for a Viennese attorney. The commissioner was certainly the attorney’s client Franz Count von Walsegg. Walsegg’s wife had died earlier in the year, and he wanted to give her a musical memorial. Whether Mozart, as his health deteriorated, saw the Requiem as his own musical memorial is difficult to say.

In any case, when he died, on December 5^th, the Requiem was a work in progress. Mozart had received part of his fee; Constanze, a widow with young children, badly needed the remainder, so she turned first to Joseph Eybler, who quickly gave up, and then several others before settling on Süssmayr. He completed the Requiem; it was delivered to Walsegg, the fee was paid, and in December 1793, the count had the work performed, from a score written in his own hand, with the words “Requiem composto dal conte Walsegg.”

Walsegg’s was not the initial presentation of the Requiem. In January 1793, it was given at a benefit concert for Constanze and her children arranged by one of Mozart’s patrons, Baron Gottfried van Swieten. Part of it appears to have been played at Mozart’s memorial service. To her death, in 1842, Constanze maintained that the Requiem was her husband’s. Neither Süssmayr nor anyone else who might have helped with the work’s completion ever contradicted her.        

However much the Requiem may be Mozart’s in conception and spirit, the fact remains that he didn’t write down every note of it. At the time of his death, he had completed the Introit and made a full sketch for the Kyrie, the Sequentia (with the exception of the Lacrymosa), and the Offertorium. The Sanctus, the Benedictus, the Agnus Dei, and the Communio were still to come. To what extent Mozart left sketches that would have guided Süssmayr is a question that has vexed scholars for more than two centuries. A number of musicologists and composers, among them Boston Musical Intelligencer adviser Robert Levin, have made their own alternative completions of the Requiem, in some cases incorporating a sketch for a Mozart Amen fugue discovered in the 1960s to finish the Lacrymosa.

But the familiar Mozart/Süssmayr score remains extremely popular, and that’s what Nelsons performed before a packed Symphony Hall Thursday. He also chose to use, as he did for February’s Bach B-minor Mass, a modest orchestra and a large chorus. Performance practice in the Requiem is variable. On his 1991 AliaVox recording, Jordi Savall leads an ensemble of 50 (orchestra, chorus, and soloists) in an intimate, dry-eyed interpretation that zips by in 46 minutes. At the other end of the spectrum, Karl Böhm on his 1971 recording for Deutsche Grammophon takes a stately 64 minutes with a chorus that, by the sound of it, must have included every able-voiced resident of Vienna.    

Nelsons had an orchestra of close to 50 and a Tanglewood Festival Chorus, under guest conductor James Bagwell, of 127 — even bigger than the chorus for the Bach. Like his B-minor Mass, Nelsons’s Requiem was fervent rather than prayerful, running just 49 minutes, with huge contrasts in dynamics and tempo. Like the Mozart concerto he’d just performed, it wasn’t particularly dark, even though the score calls for only lower winds (basset horn and bassoon) and forgoes the French horns.

The opening “Requiem aeternam” was gorgeous if otherwise unsurprising, with the TFC singing on book and Nelsons taking the decibel level way down for the “luceat eis.” The soprano, Lucy Crowe, brought rays of sunshine, if not a plethora of consonants, to the “Te decet hymnus.” It was a reverent, gratifying start, but Nelsons then tore into an imploring Kyrie and followed that, the TFC hardly having a chance to catch its breath, with a Dies irae that inspired fear and trembling.

That was the pattern for the rest of the performance. A big Rex tremendae and a powerful Confutatis were offset by a hushed, slow, swaying Lacrymosa and a serene, celestial Hostias. The Domine Jesu flew like the proverbial bat out of hell; the Sanctus was awe-struck, the Agnus Dei turbulent.

In a Requiem where the soloists don’t have a whole lot to do, Nelsons’s foursome did it well enough. My favorite of the lot was the mezzo, Tamara Mumford, whose rich voice just kept blooming. Crowe was earnest throughout, pleasing, a little strained at the top of the Lux aeterna. Bass Morris Robinson started the Tuba mirum with depth and resonance; he grew less resonant in the course of the evening, as if his voice had tightened up. Tenor Ben Johnson had the best enunciation; he was effective singing on his own but tended to get swallowed up as part of the quartet.

Enunciation from the TFC wasn’t as crisp as it had been in the B-minor Mass, but it didn’t seem to matter. It’s the chorus that talks to God in the Requiem, and this chorus, at Nelsons’s behest, had plenty to say, now pleading, now demanding, now repentant, now just plain hopeful — all the while insisting on its promised right to salvation. The fugues were energetic and extroverted, particularly the “quam olim Abrahae promisisti” that Mozart places at the end of the Domine Jesu and repeats after the Hostias, as if God needed reminding of that promise. Thursday’s audience, on the other hand, needed no reminding of the excellence of this performance.  

Nelsons, Lupu, and the BSO will repeat this program Saturday April 22 at 8 p.m.   

Jeffrey Gantz has been writing about music, dance, theater, art, film, and books for the past 35 years, first for the Boston Phoenix and currently for the Boston Globe.