The metal to insulator transition (MIT) in the charge transfer NiS{2-x}Se{x} compound has been investigated through infrared reflectivity. Measurements were performed vs applied pressure in pure NiS2 and as a function of the Se content in NiS{2-x}Se{x}. Either the compressed (pressure) or expanded (Se-alloying) lattice sustain a strongly correlated metallic state showing coherent and incoherent electronic excitations at the Fermi energy. Our theoretical analysis, pointing out the main role played by electronic correlations in the MIT, explains why both the compression and the expansion of the lattice induce a metallic state, albeit through two different microscopic mechanisms.