Remember asking my chemistry teacher this question in 1979/80: in the reduction of 1,3-dinitrobenzene with hydrogen sulphide/ aqueous ammonia, why is only one nitro group reduced?. He said that the reaction must be driven by the electon-depleted ring (two nitro groups); as one nitro-group becomes an amino, electron density in the ring increases, by definition; therefore, the reaction stops.
This is almost certainly true; but, is there a more fulsome explanation? Attempts to find the mechanism, on the net, have established that it has not yet been elucidated.
Any thoughts; theories; ideas; or, is this a worthy Ph.D. project for an ambitious young student?