Effect of selenium doping on the superconductivity of Nb<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msub><mml:mrow/><mml:mn>2</mml:mn></mml:msub></mml:math>Pd(S<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msub><mml:mrow/><mml:mrow><mml:mn>1</mml:mn><mml:mo>−</mml:mo><mml:mi>x</mml:mi></mml:mrow></mml:msub></mml:math>Se<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msub><mml:mrow/><mml:mi>x</mml:mi></mml:msub></mml:math>)<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msub><mml:mrow/><mml:mn>5</mml:mn></mml:msub></mml:math>

We study the isovalent substitution effect by partially introducing Se on the S site in the newly discovered superconductor Nb${}_{2}$PdS${}_{5}$ (${T}_{c}\ensuremath{\sim}$ 6 K) whose upper critical field is found to be far above its Pauli paramagnetic limit. In this Nb${}_{2}$Pd(S${}_{1\ensuremath{-}x}$Se${}_{x}$)${}_{5}$ (0 $\ensuremath{\le}$ $x$ $\ensuremath{\le}$ 0.8) system, superconductivity is systematically suppressed by the Se concentration and ultimately disappears when $x\ensuremath{\ge}$ 0.5, after which a semiconductinglike ground state emerges. In spite of the considerably reduced ${T}_{c}$ with Se doping, the ratio of the upper critical field ${H}_{c2}$ to ${T}_{c}$ remains unaffected. Moreover, the size of the heat capacity jump at ${T}_{c}$ is smaller than that expected for a BCS superconductor, implying that a strong-coupling theory cannot be the origin of this large upper critical field. In addition, the low-lying quasiparticle excitations are consistent with a nodeless gap opening over the Fermi surface. These results combined impose severe constraints on any theory of exotic superconductivity in this system.