Ligands can be attached in many different ways

Transition metals can have a number of ligands attached to them and each ligand can be attached in more than one place. This affects the reactivity of the ligand and the metal because each additional point of attachment means the donation of more electrons. We can show the number of atoms involved in bonding to the metal by a hapto number η. A simple Grignard reagent is η1 (pronounced ‘eta-one’) as the magnesium is attached only to one car bon atom. A metal–alkene complex is η2 because both carbon atoms of the alkene are equally involved in bonding to the metal. In these cases the η designation is not very informative as there are no alternatives, and it is usually omitted.

The bonding in these two complexes is very different. In the fi rst there is a simple σ bond between the metal and the alkyl group as in a Grignard reagent R–MgBr and this type of complex is called a σ complex. In the alkene complex, bonding is to the p orbitals only. There are no σ bonds to the metal, which sits in the middle of the π bond in between the two p orbitals. This type of complex is called a π complex.

These labels are useful where there is a choice of type of bonding, as with allylic ligands. The metal can either form a σ bond to a single carbon (hence η1), or form a π complex with the p orbitals of all three carbons of the allyl system—this would be η3. If the π complex is made from an allyl cation, the ligand has two electrons, but it has four if it is made from an allyl anion. Similarly, a cyclopentadienyl anion can act as a σ ligand (η1), an allyl ligand (η3), or, most usually, as a cyclopentadienyl ligand (η5). The distinction is very important for electron counting as these three different situations contribute two, four, or six electrons, respectively, to the complex.

Neutral ligands can also bond in a variety of ways. Cyclooctatetraene can act as an alkene (η2), a diene (η4), a triene (η6), or a tetraene (η8), and the reactivity of the ligand changes accordingly. These are all π complexes with the metal above or below the black portion of the ring and with the thick bond to the metal at right angles to the alkene plane.

To determine the number of electrons around the transition metal in a complex the valence electrons from the metal ion are added to those contributed by all the ligands. The numbers of electrons donated by various classes of ligands are summarized in the table. Anions such as halides, cyanide, alkoxide, hydride, and alkyl donate two electrons, as do neutral ligands with a lone pair such as phosphines, amines, ethers, sulfides, carbon monoxide, nitriles, and isonitriles. Unsaturated ligands can contribute as many as eight electrons and can be neutral or negatively charged. If the overall total is 18, then the complex is likely to be stable. If the overall total is less than 18 the complex is called coordinatively unsaturated.

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شعبة متابعة الأداء تنظم محاضرة تربوية لمتطوعات مجمع العلقمي استعدادًا لزيارة الأربعين

شعبة الحرم الشريف تكثف استعداداتها لتقديم خدمات متنوعة لزائري الأربعين

قسم الشؤون الفكرية يفتتح محطتين ثقافيتين في ذي قار لخدمة زائري الأربعين

العتبة العباسية المقدسة تعقد ملتقى خاصًا لمنتسبيها العاملين في مراكز إرشاد التائهين خلال زيارة الأربعين

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الآخبار الصحية

علامات فارقة بين النوبة أو السكتة القلبية.. ماهي؟

الفلفل الأحمر.. سعرات حرارية منخفضة وفوائد صحية عديدة

8 أعراض إذا شعرت بها.. اذهب للطوارئ فورا !

تعرف على أكثر 8 أماكن اتساخا في المنزل

المزيد

الآخبار التكنلوجية

إنتاج الهيدروجين الأخضر في البحر.. هذه أحدث الدراسات

"ناسا" تخطط لبناء مفاعل نووي على سطح القمر !

ابتكار ومواهب وأموال.. مارك زوكربيرغ يعلن الحرب على أبل

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

المزيد

مواضيع ذات صلة

Bonding and reactions in transition metal complexes

The 18-electron rule

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