Spectroscopic properties of carbonyl compounds

Key points: The CO stretching frequency is decreased when it serves as a π acceptor; donor ligands cause the CO stretching frequency to decrease as they supply electrons to the metal; 13C-NMR is of less use as many carbonyl compounds are fluxional on the NMR timescale.

Infrared and 13C-NMR spectroscopy are widely used to determine the arrangement of atoms in metal carbonyl compounds, as separate signals are observed for inequivalent CO ligands. NMR spectra generally contain more detailed structural information than IR spec tra provided the molecule is not fluxional (the timescales of the NMR and IR transition are different, Sections 8.4, 8.5). However, IR spectra are often simpler to obtain, and are particularly useful for following reactions. Most CO stretching bands occur in the range 2100 1700 cm1, a region that is generally free of bands due to organic groups. Both the range of CO stretching frequencies (see Fig. 22.4) and the number of CO bands (Table 22.7) are important for making structural inferences. Group theory allows us to predict the number of active CO stretches in both the IR and Raman spectra (Section 6.5). If the CO ligands are not related by a centre of inversion or a threefold or higher axis of symmetry, a molecule with N CO ligands will have N CO stretching absorption bands. Thus, a bent OCMCO group (with only a twofold symmetry axis) will have two infrared absorptions because both the symmetric (69) and antisymmetric (70) stretches cause the electric dipole moment to change and are IR active. Highly symmetrical molecules have fewer bands than CO ligands. Thus, in a linear OC MCO group, only one IR band (corresponding to the out-of-phase stretching of the two CO ligands) is observed in the CO stretching region because the symmetrical stretch leaves the overall electric dipole moment unchanged. As shown in Fig. 22.12, the positions of the CO ligands in a metal carbonyl may be more symmetrical than the point group of the whole compound suggests and then fewer bands will be observed than are predicted on the basis of the overall point group. Raman spectroscopy can be very useful in assign ing structures because the selection rules complement those of IR (Sections 6.5 and 8.4). Thus, for a linear OCMCO group, the symmetrical stretching of the two CO ligands is observed in the Raman spectrum. As we noted in Section 22.5, infrared spectroscopy is also useful for distinguishing terminal CO (MCO) from bridging CO (μ2-CO) and face-bridging CO (μ3-CO). It can also be used to determine the order of π-acceptor strengths for the other ligands present in a complex. A motionally averaged NMR signal is observed when a molecule undergoes changes in structure more rapidly than the technique can resolve (Section 8.5). Although this phenomenon is quite common in the NMR spectra of organometallic compounds, it is not normally observed in their IR or Raman spectra. An example of this difference is Fe (CO)₅,

Figure 22.12 The infrared spectrum of [Fe2 (Cp)₂(CO)₄]. Note the two high-frequency terminal CO stretches and the lower frequency absorption of the bridging CO ligands. Although two bridging CO bands would be expected on account of the low symmetry of the complex, a single band is observed because the two bridging CO groups are nearly collinear.

for which the ¹³C-NMR signal shows a single line at Ϭ=210, whereas IR and Raman spectra are consistent with a trigonal-bipyramidal structure.

اخر الاخبار

اخبار العتبة العباسية المقدسة

لزيادة المساحات الخضراء.. قسم الحزام الأخضر الثاني يشرع بحملة تشجير في جامعة العميد

اللجنة التحضيرية لمهرجان روح النبوة تعلن تمديد موعد تسلم البحوث الخاصة بالمسابقة البحثية

المجمع العلمي يطلق جلستين قرآنيتين لطلبة العلوم الدينية في النجف

قسم الشؤون الفكرية يقدم محاضرة حول أساليب الحوار الإيجابي لوفد شبابي من البصرة

المزيد

الآخبار الصحية

هل ممارسة الرياضة على معدة فارغة تحرق الدهون أكثر؟

العلم يوضح الوقت "الأمثل" لشرب القهوة

دراسة: "طريقة سهلة" لحماية رئتيك من آثار تلوث الهواء

بالذكاء الاصطناعي.. أداة جديدة لتحسين علاج الصرع عند الأطفال

المزيد

الآخبار التكنلوجية

إيلون ماسك يعتزم إطلاق "غروكيبيديا" كمنافس لـ "ويكيبيديا"

"قرار" من أبل لحماية هواتف آيفون 17 من الخدوش

ميزة جديدة على "تشات جي بي تي".. شراء المنتجات من المحادثة

الصين تكشف عن توربين رياح طائر يغير قواعد الطاقة المتجددة

المزيد

مواضيع ذات صلة

Isolobal analogies

Structure of clusters

Bent metallocene complexes

Fluxional behaviour of metallocenes

Bonding in bis(cyclopentadienyl)metal complexes

Synthesis and reactivity of cyclopentadienyl compounds

Reactions of the CO ligand

Metal carbonyl basicity

Oxidation and reduction of carbonyls

Properties of homoleptic carbonyls

Synthesis of homoleptic carbonyls

Homoleptic carbonyls