We describe an unusual in-plane type of porphyrin core distortion, tetragonal elongation (TE), observed experimentally in diboron porphyrins. The vibrational spectra of several of these complexes exhibit shifts that we have assigned to this TE distortion by comparing experimental spectra with DFT computational findings. The influence of TE in porphyrin systems was isolated using DFT analysis of the well-known model compounds Ni(II)porphine and Zn(II)porphine, with the macrocycle ring constrained to eliminate the influence of out-of-plane (OOP) distortions. A significant down-shift in frequencies was observed for porphyrin normal vibrational modes, particularly the in-plane A(1g)/B-1g modes that are dominated by contributions from stretching and bending of C-alpha-C-m coordinates. In contrast, TE had little effect on the nu(Pyr(halfring)) and delta(Pyr(def)) modes, though the lowered symmetry of the system resulted in significant splitting of the B-2u and B-3u modes. The impact of the TE distortion upon the diboron porphyrin vibrational spectrum was probed experimentally using Raman spectroscopy of B2O2(BCl3)(2)(TTP), B2OF2(TTP), and B2OPhOH2(TTP) (TTP = 5,10,15,20-(tetra-p-tolyl)porphyrin). Comparing the experimentally obtained spectral signatures to the computational findings allowed us to assign the large shifts observed for the v(2) and v(3) modes to the TE distortion in diboron porphyrins.