Five Coordination (ML5)

Examples of ML5 are found for all of the first-row transition metal ions, as well as some other metal ions. Although once considered rare, growth in coordination chemistry has led to five-coordination becoming met almost as frequently as four-coordination. This exhibits one of the limitations of making comparisons of this type; rarity may not be a result of any inherent restriction, but may simply reflect limited experimental development. Given that four-coordination is common and six-coordination very common, it is perhaps not surprising to find five-coordination also having matching status, at least for lighter, smaller metal ions. Five-coordination is not commonly met in complexes of the heavier transition metals, however. The amended VSEPR model predicts two forms of five-coordination, and experimental chemistry has clearly identified many examples of both forms. These limiting structures are square-based pyramidal (or, simply, square pyramidal) and trigonal bipyramidal (Figure 4.13). The classical square-based pyramidal shape is formed simply by cleaving off one bond Froman octahedral shape which leaves the metal in the same plane as the four-square based ligands. In reality almost no complexes exhibit this shape, but rather adopt a distorted

Figure 4.13

The two limiting shapes for five-coordination.

square pyramidal shape where the metal lies above the basal square plane, with typically the angle from the apical donor through the metal to each donor in the base around 105◦ instead of 90◦. Considering electron pair repulsion alone, this distorted shape (distorted only in terms of themetal location; the Egyptian pyramid shape created by the donor groups is otherwise unaltered) is actually more stable than the form created by simply truncating an octahedron, and is only slightly less stable than the trigonal bipyramidal geometry. As a consequence, it has become usual to regard the square pyramidal shape as that with the metal above the pyramidal base plane and you will see it represented in this way almost exclusively.

Once again, as described for three- and four-coordination, it is possible to convert from one form to the other through bond angle changes without any bond-breaking. Both geometries are common, but in practice there are many structures that are intermediate between these two. The two limiting structures are of similar energy and as predicted, some complexes display an equilibrium between the two; [Ni(CN)5]3- for example, crystallizes with both structural forms of the anion present in the crystals. If we examine the two five-coordinate shapes from a crystal field perspective, the d orbitals split in a different way to that found for octahedral, tetrahedral and square planar shapes since the d orbitals find the ligands in clearly different locations in space. The crystal field splitting pattern for the two is shown in Figure 4.14. From this pattern, crystal field stabilization energies can be calculated, and favour the square pyramidal geometry in all cases (apart from the trivial situations d0 and d10) except for high spin d5. This prediction differs from the outcome from the electron pair repulsion model.

Although electron pair repulsion and crystal field stabilization energy (CFSE) influences operate, it appears nevertheless that the steric or shape demands of at least polydentate ligands play a dominant influence on complex shape. This is exemplified in Figure 4.15, where an example of a ‘three-legged’ ligand shape fits best to the trigonal bipyramidal geometry, occupying the top four positions of the complex shape, with a fifth simple ligand then occupying the underside– the whole assembly looks a little like an open umbrella. The ligand is pre-disposed to this shape with limited steric interaction when bound. Likewise, the cyclic tetraamine represented in Figure 4.15 is predisposed to binding in the square pyramidal shape.

Overall trigonal bipyramidal is found rather than square pyramidal shape except where steric requirements of polydentate ligands are important or where bonding occurs (as in vanadyl complexes [VOL₄] where the VO bond occupies the axial site with four other ligands in the basal plane). However, exceptions abound, reflecting the similar energies of the two forms. Examples of complexes with trigonal bipyramidal geometry are

Figure 4.14: Crystal field splitting pattern for trigonal bipyramidal (left) and square pyramidal (right) ML₅ complexes.

[Fe(CO)₅] [Co(NCCH₃)5] + [NiBr₃(PEt₃)₂] and [CuCl₅]₃-; square pyramidal shapes occur   for [Ni(CN)₅]₃−[VO(SCN)₄][ReBr₄O]-and [Fe(NO)(S₂CNEt₂)₂] (Figure 4.16).

Whereas a limited number of complexes display equivalent lengths for all bonds, it is more common for some distortions from the regular stereochemistry to be found both in terms of bond distances and angles. For example in trigonal bipyramidal [Co (NCCH₃)₅] + the two axial Co-N bonds are5 pm longer than the three equatorial bonds and in [CuCl₅]₃- axial bonds are 95 pm shorter than equatorial bonds. For square pyramidal complexes the

Figure 4.15 Ligand shape directing complex shape in five-coordination.

Figure 4.16 : Examples of complexes adopting one of the two shapes of five-coordination.

metal lies typically between 30 and 50 pm above the square plane of donors, close to the value of 48 pm calculated from geometry assuming an apical donor–metal–basal donor angle of 104◦ for 200 pm metal–donor bonds. In this stereochemistry, the single axial bond tends to be longer than the four equatorial bonds; for [Ni(CN)₅]₃-the former is 217 pm and the latter are 187 pm.

اخر الاخبار

اخبار العتبة العباسية المقدسة

طوال الشهر الفضيل.. قسم الشؤون الخدمية يقدم أكثر من 42 ألف صندوق ماء للزائرين

بمشاركة 142 منتسبًا.. قسم الشؤون الدينية يختتم دورته الفقهية التطويرية السادسة

قسم بين الحرمين: قدمنا أكثر من 50 ألف وجبة إفطار للزائرين خلال شهر رمضان المبارك

المجمع العلمي يختتم نشاطاته القرآنية الرمضانية في المثنى

المزيد

الآخبار الصحية

كوب قبل النوم.. مشروب طبيعي يحارب الأرق

دون أدوية.. طريقة طبيعية لخفض ضغط الدم بفعالية مذهلة

المعكرونة ليست عدوك.. كيف تصبح جزءا من وجبة صحية متوازنة؟

دراسة: تناول القهوة يوميا يخفض خطر الإصابة بالاضطرابات النفسية

المزيد

الآخبار التكنلوجية

علماء روس يخططون لبناء منشأة للكشف عن المادة المظلمة

ثاني أكسيد الكربون ليس تهديدا للمناخ فقط.. بل لصحة الإنسان أيضا!

ديدان الأرض تكشف تلوث التربة بالمعادن الثقيلة

خبير أرصاد جوية: ظاهرة النينيو القادمة قد تكون الأقوى خلال 10 سنوات

المزيد

مواضيع ذات صلة

Constitutional (Structural) Isomerism

Isomerism– Real 3D Effects

Chameleon Complexes

Moulding a Relationship - Ligand Influences

Metallic Genetics– Metal Ion Influences

Higher Coordination Numbers (ML7 to ML9)

Six Coordination (ML6)

Four Coordination (ML4)

Three Coordination (ML3)

Two Coordination (ML2)

Complex Lifetimes– Together Forever

?Preferences– Do You Like What I Like