The 18-electron rule

There is a contradiction in what is required of a metal complex for it to be useful to us. Initially, it will need to be stable and have a long enough lifetime to enable study and, ideally, storage. But once it enters the reaction vessel, stability is a disadvantage: instead we want reactivity. Our ideal catalyst is a complex that is stable in the resting state, but quickly becomes activated in solution—perhaps by loss of a ligand—so that it can interact with the substrate. Fortunately, there is a simple guide to the stability of transition metal complexes: the 18-electron rule. If a complex satisfi es the 18-electron rule it means that the metal at the centre of the complex has the noble gas configuration of 18 electrons in the valence shell, and the complex is likely to be stable. The requirement for 18 electrons comes from the need to fill one ‘s’ orbital, five ‘d’ orbit als, and three ‘p’ orbitals with two electrons in each. The 18 electrons we need can come from those the metal already possesses plus those donated by any coordinating ligands. The table below gives you the number of valence electrons each metal starts with before it acquires any ligands. Notice that the ‘new’ group numbers 1–18 give you the answer without any calculation. The most important are highlighted.

Metals to the left-hand side of this list obviously need many more electrons to make up the magic 18. Chromium, for example, forms stable complexes with a benzene ring, giving it six electrons, and three molecules of carbon monoxide, giving it two each: 6 + 6 + 2 + 2 + 2 = 18. Palladium is happy with just four triphenylphosphines (Ph3P:) giving it two each: 10 + 2 + 2 + 2 + 2 = 18. You may already know from your study of inorganic chemistry that there are exceptions to the 18-electron rule, particularly among complexes of Ti, Zr, Ni, Pd, and Pt, which can all form stable 16-electron complexes. The important 16-electron Pd(II) complex with two chlorides and two acetonitriles (MeCN) will feature heavily in this chapter. The so-called platinum metals Ni, Pd, and Pt are extremely important in catalytic processes, as you will see later on. Their stable 16-electron configuration results from a high-energy vacant orbital caused by the complex adopting a square planar geometry.

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علامات فارقة بين النوبة أو السكتة القلبية.. ماهي؟

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8 أعراض إذا شعرت بها.. اذهب للطوارئ فورا !

تعرف على أكثر 8 أماكن اتساخا في المنزل

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عودة التعاون الفضائي بين روسيا وأميركا.. وبحث ملفات "هامة"

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مواضيع ذات صلة

Bonding and reactions in transition metal complexes

Ligands can be attached in many different ways

Transition metals extend the range of organic reactions

The Ritter reaction and the Beckmann fragmentation

Specific base catalysis

Specific acid catalysis

Entropy of activation

The kinetic isotope effect

Non-linear Hammett plots

A complete picture of the transition state from Hammett plots

Using the Hammett ρ values to uncover mechanisms

Reactions with small Hammett ρ values