Magnetic measurements

Key points: Magnetic measurements are used to determine the number of unpaired spins in a complex and hence to identify its ground-state configuration. A spin-only calculation may fail for low-spin d⁵ and for high-spin 3d⁶ and 3d⁷ complexes. The experimental distinction between high-spin and low-spin octahedral complexes is based on the determination of their magnetic properties. Compounds are classified as diamagnetic if they are repelled by a magnetic field and paramagnetic if they are attracted by a magnetic field. The two classes are distinguished experimentally by magnetometry. The magnitude of the paramagnetism of a complex is commonly reported in terms of the magnetic dipole moment it possesses: the higher the magnetic dipole moment of the complex, the greater the paramagnetism of the sample. In a free atom or ion, both the orbital and the spin angular momenta give rise to a magnetic moment and contribute to the paramagnetism. When the atom or ion is part of a complex, any orbital angular momentum is normally quenched, or suppressed, as a result of the interactions of the electrons with their nonspherical environment. However, if any electrons are unpaired the net electron spin angular momentum survives and gives rise to spin-only paramagnetism, which is characteristic of many d-metal complexes. The spin only magnetic moment, μ, of a complex with total spin quantum number S is

Where μ_B is the Bohr magneton, μB= eh/2me with the value 9.274 10 24 J T1. Because S=1/2 N, where N is the number of unpaired electrons, each with spin s=1/2,

Ameasurement of the magnetic moment of a d-block complex can usually be interpreted in terms of the number of unpaired electrons it contains, and hence the measurement can be used to distinguish between high-spin and low-spin complexes. For example, magnetic measurements on a d6 complex easily distinguish between a high-spin t2g⁴ eg² (N=4, S=2, μ=4.90μB) configuration and a low-spin t2g 6 (N=0, S=0, μ=0) configuration. The spin-only magnetic moments for some electron configurations are listed in Table 20.3 and compared there with experimental values for a number of 3d complexes. For most 3d complexes (and some 4d complexes), experimental values lie reasonably close to spin-only predictions, so it becomes possible to identify correctly the number of unpaired electrons and assign the ground-state configuration. For instance, [Fe (OH₂)₆]³⁺ is paramagnetic with a magnetic moment of 5.9μB. As shown in Table 20.3, this value is consistent with there be ing five unpaired electrons (N=5 and S=5/2 ), which implies a high-spin t_2g³ eg² configuration.

The interpretation of magnetic measurements is sometimes less straightforward than this example might suggest. For example, the potassium salt of [Fe(CN)₆]^3- has μ=2.3μ_B, which is between the spin-only values for one and two unpaired electrons (1.7μB and 2.8μB , respectively). In this case, the spin-only assumption has failed because the orbital contribution to the magnetic moment is substantial.

For orbital angular momentum to contribute, and hence for the paramagnetism to dif fer significantly from the spin-only value, there must be one or more unfilled or half-filled orbitals similar in energy to the orbitals occupied by the unpaired spins and of the appropriate symmetry (one that is related to the occupied orbital by rotation round the diretion of the applied field). If that is so, the applied magnetic field can force the electrons to circulate around the metal ion by using the low-lying orbitals and hence it generates orbital angular momentum and a corresponding orbital contribution to the total magnetic moment (Fig. 20.6). Departure from spin-only values is generally large for low-spin d5 and for high-spin 3d6 and 3d7 complexes. It is also possible for the electronic state of the metal ion to change (for example with temperature), leading to a change from high-spin to low-spin and a change in the magnetic moment. Such complexes are referred to as spin-crossover complexes and are discussed in more detail, together with the effects of cooperative magnetism, in Sections 20.8 and 20.9.

Figure 20.6 If there is a low-lying orbital of the correct symmetry, the applied field may induce the circulation of the electrons in a complex and hence generate orbital angular momentum. This diagram shows the way in which circulation may arise when the field is applied perpendicular to the xy-plane (perpendicular to this page).

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اخبار العتبة العباسية المقدسة

جامعة العميد تنظم ورشة تدريبية عن برنامج منصة التعليم العالي

قسم الشؤون الفكرية ينظم برنامجًا دينيًّا لكشافة الكفيل في النجف الأشرف

العتبة العباسية المقدسة تقدم دعوة لجامعتي الكوفة والطوسي للمشاركة في مشروعها القرآني الخاص بطلبة الجامعات

قسم الشؤون الفكرية يصدر مجموعة جديدة من الإصدارات المعرفية والثقافية

المزيد

الآخبار الصحية

كيف يمكن لحفنة من الجوز أن تؤثر على نومك وتركيزك؟

دراسة: كوب ميلك شيك واحد قد يكون "قنبلة دماغية"

هل يعد "الباراسيتامول" خيارا آمنا للنساء الحوامل؟

دراسة تكشف الرابط بين العلاقات العاطفية والرضا عن الحياة

المزيد

الآخبار التكنلوجية

كارثة صامتة.. "الهواء الملوث" يسرق نور عيون الأطفال

دراسة: رصد أول دليل على وجود "إشعاع هوكينغ"

رسالة بلغة لا تعرفها؟.. "واتساب" يترجمها لك فورا

"إنستغرام".. ميزة جديدة لكشف "الكذب في العمر"

المزيد

مواضيع ذات صلة

Thermochemical correlations

Octahedral complexes

Sulfide complexes

Disulfides

Polyoxometallates

Mononuclear oxido complexes

Metal oxides

Structural trends

Oxidation states down a group

Intermediate oxidation states in the 4d and 5d series

Intermediate oxidation states in the 3d series

High oxidation states