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The spin\u2013orbit coupling (SOC)-induced changes in the T\u2013X equilibrium bond lengths exhibit a non-monotonic pattern across the 15 TX4 compounds, governed by the competition between central-atom p1\/2 contraction and ligand p3\/2 elongation. The natural electron configuration-based spinor occupation inference (NSOI) framework, extended from diatomic to polyatomic systems, accounts for all observed structural variations, including the counterintuitive enhancement of bond contraction from TF4 to TCl4 and the crossover from contraction to elongation in PbI4. For the TX2 series, the NSOI framework consistently rationalizes not only the SOC-induced bond length changes but also the bond angle changes, which are interpreted as secondary geometric consequences of changes in bond lengths and X\u2013X interactions. The robustness of the NSOI approach was confirmed with the MN15 functional. The reaction energies for TX4 \u2192 TX2 + X2 decrease systematically from Ge to Pb, consistent with the inert-pair effect. Time-dependent density functional theory calculations reveal that SOC activates singlet\u2013triplet mixing, transforming the UV\u2013Vis absorption spectra of heavy-atom TX4 and providing theoretical reference data for experimentally elusive species such as PbX4. In contrast to the structural SO effects, which are attenuated by partial cancellation between central-atom and ligand contributions, the spectral SO effects are more pronounced because singlet\u2013triplet mixing introduces absorption features absent in the scalar-relativistic spectra.<\/jats:p>","DOI":"10.1063\/5.0329963","type":"journal-article","created":{"date-parts":[[2026,6,1]],"date-time":"2026-06-01T11:17:53Z","timestamp":1780312673000},"update-policy":"https:\/\/doi.org\/10.1063\/aip-crossmark-policy-page","source":"Crossref","is-referenced-by-count":0,"title":["Spin\u2013orbit effects on the molecular properties of Group 14 tetracoordinate compounds TX4 (T = Ge, Sn, and Pb; X = H, F, Cl, Br, and I): Natural electron configuration-based rationalization of structural variations"],"prefix":"10.1063","volume":"164","author":[{"ORCID":"https:\/\/orcid.org\/0000-0002-7783-0200","authenticated-orcid":false,"given":"Joonghan","family":"Kim","sequence":"first","affiliation":[{"name":"Department of Chemistry, The Catholic University of Korea 1 , Bucheon 14662,","place":["Republic of Korea"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0009-0008-8256-4155","authenticated-orcid":false,"given":"Yurim","family":"Jin","sequence":"additional","affiliation":[{"name":"Department of Chemistry, The Catholic University of Korea 1 , Bucheon 14662,","place":["Republic of Korea"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0009-0000-8462-8353","authenticated-orcid":false,"given":"Wonil","family":"Seo","sequence":"additional","affiliation":[{"name":"Department of Chemistry, The Catholic University of Korea 1 , Bucheon 14662,","place":["Republic of Korea"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Hyeonji","family":"Kim","sequence":"additional","affiliation":[{"name":"Department of Chemistry, The Catholic University of Korea 1 , Bucheon 14662,","place":["Republic of Korea"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0009-0009-8905-7401","authenticated-orcid":false,"given":"Ingyeong","family":"Kim","sequence":"additional","affiliation":[{"name":"Department of Chemistry, The Catholic University of Korea 1 , Bucheon 14662,","place":["Republic of Korea"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-5320-5259","authenticated-orcid":false,"given":"Rajesh K.","family":"Yadav","sequence":"additional","affiliation":[{"name":"Department of Chemistry and Environmental Science, Madan Mohan Malaviya University of Technology 2 , Gorakhpur 273010, Uttar Pradesh,","place":["India"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-7326-1725","authenticated-orcid":false,"given":"Jeong Sik","family":"Lim","sequence":"additional","affiliation":[{"name":"Semiconductor and Display Metrology Group, Korea Research of Institute of Standard and Science (KRISS) 3 , 267 Gajeong-ro, Yuseong-gu, Daejeon 34113,","place":["Republic of Korea"]},{"name":"Precision Measurement Engineering, University of Science and Technology (UST) 4 , Daejeon 34113,","place":["Republic of Korea"]}],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"317","published-online":{"date-parts":[[2026,6,1]]},"reference":[{"key":"2026060107174648200_c1","doi-asserted-by":"publisher","first-page":"223","DOI":"10.1007\/s11090-017-9852-4","article-title":"Removal of tin from extreme ultraviolet collector optics by in-situ hydrogen plasma etching","volume":"38","year":"2018","journal-title":"Plasma Chem. 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