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In a joint experimental and theoretical study, we investigate the isostructural collapse from the ambient pressure tetragonal phase to a collapsed tetragonal phase for nonsuperconducting metallic SrFe2As2 and SrFe1.8Ru0.2As2. The crystallographic details have been studied using x-ray powder diffraction up to 20 GPa pressure in a diamond anvil cell. The structural phase transition occurs at 10 and 9 GPa for SrFe2As2 and SrFe1.8Ru0.2As2, respectively. The changes in the unit cell dimensions are highly anisotropic with a continuous decrease of the c lattice parameter with pressure, while the a-axis length increases until the transition to a collapsed tetragonal phase and then continues to decrease. Across the phase transition, we observe volume reductions of 5% and 4% for SrFe2As2 and SrFe1.8Ru0.2As2, respectively. We are able to discern that Ru substitution on the Fe site acts like "chemical pressure" to the system. Density-functional theory-based calculations of the electronic structure and electron localizability indicator are consistent with the experimental observations. Detailed analysis of the electronic structure in k space and real space reveals As 4pz interlayer bond formation as the driving force of the c/a collapse with a change in the As-As bond length of about 0.35 Å. [ABSTRACT FROM AUTHOR]