We have investigated the property of the non-magnetic insulating state of a pressure-induced molecular superconductor with a quasi-triangular lattice consisting of tight dimer units applying vibrational and reflectance spectroscopy to monoclinic-phase of C2H5(CH3)(3)P[Pd(dimt)(2)](2) (dmit = 1,3-dithiole-2-thione-4,5-dithiolate). From the analyses of the spectra, it is founded that two asymmetric dimers form a weakly bounded tetramer and the 2D layer is a slightly anisotropic. Theses results indicate that the cooperation between nearest Coulomb repulsions and valence bond order, which is operative for the anisotropic system of triclinic-phase of C2H5(CH3)(3)P[Pd(dimt)(2)](2), inhibits spin frustration in the monoclinic-phase. Our result also suggests that a half-filled picture does not always hold for the molecular superconductors consisting of tight dimer units.