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EDA\u2010NOCV on the sixteen <jats:italic>LS<\/jats:italic> complexes revealed that neither <jats:italic>\u0394E<\/jats:italic><jats:sub><jats:italic>orb,\u03c3+\u03c0<\/jats:italic><\/jats:sub> (R<jats:sup>2<\/jats:sup>=0.48) nor <jats:italic>\u0394E<\/jats:italic><jats:sub><jats:italic>orb<\/jats:italic>,<jats:italic>\u03c0<\/jats:italic><\/jats:sub> (R<jats:sup>2<\/jats:sup>=0.31) correlated with the experimental solution T<jats:sub>1\/2<\/jats:sub> values (which are expected to reflect the ligand field imposed on the iron centre), but that <jats:italic>\u0394E<\/jats:italic><jats:sub><jats:italic>orb<\/jats:italic>,<jats:italic>\u03c3<\/jats:italic><\/jats:sub> correlates well (R<jats:sup>2<\/jats:sup>=0.82) and implies that as <jats:bold>X<\/jats:bold> changes from <jats:bold>EDG<\/jats:bold>\u2192<jats:bold>EWG<\/jats:bold> (Electron Donating to Withdrawing Group), the ligand becomes a better \u03c3\u2010donor. This counter\u2010intuitive result was further probed by Mulliken analysis of the <jats:bold>N<jats:sub>A<\/jats:sub><\/jats:bold> atomic orbitals: <jats:bold>N<jats:sub>A<\/jats:sub><\/jats:bold>(<jats:italic>p<jats:sub>x<\/jats:sub><\/jats:italic>) involved in the Fe\u2212N \u03c3\u2010bond vs. the perpendicular <jats:bold>N<jats:sub>A<\/jats:sub><\/jats:bold>(<jats:italic>p<jats:sub>z<\/jats:sub><\/jats:italic>) employed in the ligand aromatic \u03c0\u2010system. As <jats:bold>X<\/jats:bold> changes <jats:bold>EDG<\/jats:bold>\u2192<jats:bold>EWG<\/jats:bold>, the electron population on <jats:bold>N<jats:sub>A<\/jats:sub><\/jats:bold>(<jats:italic>p<jats:sub>z<\/jats:sub><\/jats:italic>) decreases, making it a better \u03c0\u2010acceptor, whilst that in <jats:bold>N<jats:sub>A<\/jats:sub><\/jats:bold>(<jats:italic>p<jats:sub>x<\/jats:sub><\/jats:italic>) increases, making it a better \u03c3\u2010bond donor; both increase ligand field, and T<jats:sub>1\/2<\/jats:sub> as observed. In 2016, Halcrow, Deeth and co\u2010workers proposed an intuitively reasonable explanation of the effect of the <jats:italic>para<\/jats:italic>\u2010<jats:bold>X<\/jats:bold> substituents on the<\/jats:p><jats:p>T<jats:sub>1\/2<\/jats:sub> values in this family of complexes, consistent with the calculated MO energy levels, that M\u2192L \u03c0\u2010backdonation dominates in these M\u2212L bonds. Here the quantitative EDA\u2010NOCV analysis of the M\u2212L bond contributions provides a more complete, coherent and detailed picture of the relative impact of M\u2212L \u03c3\u2010versus \u03c0\u2010bonding in determining the observed T<jats:sub>1\/2<\/jats:sub>, refining the earlier interpretation and revealing the importance of the \u03c3\u2010bonding. Furthermore, our results are in perfect agreement with the \u0394E(HS\u2010LS) vs. \u03c3<jats:sub>p<\/jats:sub><jats:sup>+<\/jats:sup>(X) correlation reported in their work.<\/jats:p>","DOI":"10.1002\/chem.202104314","type":"journal-article","created":{"date-parts":[[2022,2,27]],"date-time":"2022-02-27T22:03:41Z","timestamp":1645999421000},"update-policy":"https:\/\/doi.org\/10.1002\/crossmark_policy","source":"Crossref","is-referenced-by-count":15,"title":["Quantitative Assessment of Ligand Substituent Effects on \u03c3\u2010 and \u03c0\u2010Contributions to Fe\u2212N Bonds in Spin Crossover Fe<sup>II<\/sup> Complexes"],"prefix":"10.1002","volume":"28","author":[{"given":"Luca","family":"Bond\u00ec","sequence":"first","affiliation":[{"name":"Department of Chemistry MacDiarmid Institute of Advanced Materials and Nanotechnology University of Otago  PO Box 56 Dunedin 9054 New Zealand"},{"name":"Department of Chemistry \u2018Ugo Schiff' and INSTM Research Unit University of Florence  50019 Sesto Fiorentino Italy"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Anna L.","family":"Garden","sequence":"additional","affiliation":[{"name":"Department of Chemistry MacDiarmid Institute of Advanced Materials and Nanotechnology University of Otago  PO Box 56 Dunedin 9054 New Zealand"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Federico","family":"Totti","sequence":"additional","affiliation":[{"name":"Department of Chemistry \u2018Ugo Schiff' and INSTM Research Unit University of Florence  50019 Sesto Fiorentino Italy"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Paul","family":"Jerabek","sequence":"additional","affiliation":[{"name":"Institute of Hydrogen Technology Helmholtz-Zentrum Hereon  Max-Planck-Stra\u00dfe 1 21502 Geesthacht Germany"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-5878-8238","authenticated-orcid":false,"given":"Sally","family":"Brooker","sequence":"additional","affiliation":[{"name":"Department of Chemistry MacDiarmid Institute of Advanced Materials and Nanotechnology University of Otago  PO Box 56 Dunedin 9054 New Zealand"}],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"311","published-online":{"date-parts":[[2022,3,21]]},"reference":[{"key":"e_1_2_9_1_1","unstructured":"\u00a0"},{"key":"e_1_2_9_2_2","doi-asserted-by":"publisher","DOI":"10.1039\/B701085K"},{"key":"e_1_2_9_3_2","doi-asserted-by":"publisher","DOI":"10.1039\/C3CP54028F"},{"key":"e_1_2_9_4_2","doi-asserted-by":"publisher","DOI":"10.1039\/C3TC31750A"},{"key":"e_1_2_9_5_1","unstructured":"\u00a0"},{"key":"e_1_2_9_6_2","doi-asserted-by":"publisher","DOI":"10.1039\/c3nj01255g"},{"key":"e_1_2_9_7_2","doi-asserted-by":"publisher","DOI":"10.1002\/ejic.201201151"},{"key":"e_1_2_9_8_2","unstructured":"A. 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