Phase transformations in the Gd5Si4-xPx system (0 <= x <= 2), studied through X-ray diffraction techniques, reveal an intimate coupling between the crystal structure and valence electron count. An increase in the valence electron count through P substitution results in breaking the interslab T-T dimers (d(T-T) = 3.74 angstrom; T is a mixture of Si and P) and shear movement of the (2)(infinity)[Gd5T4] slabs in Gd5Si2.75P1.25. The Gd5Si2.75P1.25 phase extends the existence of the orthorhombic Sm5Ge4-type structures to the valence electron count larger than 31 e(-)/formula unit. Tight-binding linear-muffin-tin-orbital calculations trace the origin of the T-T dimer cleavage in Gd5Si2.75P1.25 to a larger population of antibonding states within the dimers.