Tetrazole/tetrazolate monocydopentadienyliron(II) and ruthenium(II) compounds of general formulas [(eta(5)-C5H5)M(dppe)(N-4(H)CC6H4NO2)][PF6]/[(eta(5)-C5H5)M(dppe)(N4CC6H4NO2] were investigated for their pH-switching second-order nonlinear optical (SONLO) properties. Compounds [(eta(5)-C5H5)M(dppe)(N4CC6H4NO2) (M = Fe, Ru) and compound [(eta(5)-C5H5)Ru(dppe)(N-4(H)CC6H4NO2)][PF6] were fully characterized by (H-1-, C-13-, P-31-) NMR, cyclic voltammetty, and elemental analysis, and compounds [(eta(5)-C5H5)Fe(dppe)(N4CC6H4NO2)] and [(eta(5)-C5H5)Ru(dppe)(N-4(H)CC6H4NO2)][PF6] were further characterized by single-crystal X-ray diffraction; the synthesis of [(eta(5)-C5H5)Fe(dppe)(N-4(H)CC6H4NO2)][PF6] was unsuccessful. Time-dependent density functional theory calculations were performed using PBE0 and CAM-B3LYP functionals to evaluate the first hyperpolarizability (beta(tot)) of the tetrazole/tetrazolate complexes and for a detailed analysis of the experimental data. Both functionals piedict (i) high first hyperpolarizabilities for the tetrazolate complexes [(eta(5)-C5H5)M(dppe)(N4CC6H4NO2)], with beta(tot)[Ru] approximate to 1.2 beta(tot)[Fe], and (ii) a 3-fold reduction in beta(tot)[Ru] protonation, in complex [(eta(5)-C5H5)Ru(dppe)(N-4(H)CC6H4NO2)](+), forecasting [(eta(5)-C5H5)Ru(dppe)(N4CC6H4NO2)]/[(eta(5)-C5H5)Ru(dppe)(N-4(H)CC6H4NO2)](+) complexes as on/off, pH-switchable SONLO forms.