{"id":16857,"date":"2026-07-01T08:07:36","date_gmt":"2026-07-01T08:07:36","guid":{"rendered":"https:\/\/www.vedprep.com\/exams\/?p=16857"},"modified":"2026-07-01T10:21:16","modified_gmt":"2026-07-01T10:21:16","slug":"18-electron-rule","status":"publish","type":"post","link":"https:\/\/www.vedprep.com\/exams\/rpsc\/18-electron-rule\/","title":{"rendered":"18-Electron rule: Master Tips For RPSC Assistant Professor"},"content":{"rendered":"

You are diving deep into the RPSC Assistant Professor syllabus, and right there in Organometallic Chemistry, the 18-electron rule<\/strong> stands out as a high-yield topic. If you check out standard textbooks like Cotton and Wilkinson\u2019s <\/span>Advanced Inorganic Chemistry<\/span><\/i> or Miessler and Tarr’s <\/span>Inorganic Chemistry<\/span><\/i>, they spend a lot of time on this. At <\/span>VedPrep<\/b>, we always tell our students that mastering this rule is one of the easiest ways to secure solid marks in both the RPSC and CSIR NET exams.<\/span><\/p>\n

The central metal atom or ion is surrounded by ligands that donate electron pairs. The stability of these complexes depends heavily on the electronic configuration of that central metal. Because transition metals have those partially filled d subshells, they have extra room to play with. The 18-electron rule<\/strong> says that a transition metal complex reaches peak stability when it fills its valence shell with 18 electrons\u2014combining its own electrons with the ones donated by the ligands. It is the transition metal equivalent of the octet rule you learned in school.<\/span><\/p>\n

You will often hear about low-spin and high-spin configurations here. In a low-spin setup, electrons pack tightly into the lower-energy orbitals first. In a high-spin setup, they spread out into higher-energy levels. For maximum stability and to hit that 18-electron sweet spot, complexes usually favor a low-spin arrangement. Take Mo(CO)\u2086 as a quick example: the Molybdenum center brings 6 valence electrons to the table, and the 6 carbonyl (CO) ligands donate 2 electrons each. That is 6 + 12 = 18. It\u2019s perfectly stable, and it’s exactly the kind of neat package the rule predicts.<\/span><\/p>\n

Overview: 18-Electron Rule For RPSC Assistant Professor<\/b><\/h2>\n

Let\u2019s try one more practice question together. What is the oxidation state of the metal in [Fe(CO)\u2085], and does it follow our rule?<\/span><\/p>\n

Let’s break it down:<\/span><\/p>\n