Six Coordination (ML6)

ML6 is the most common coordination type by far that is met for transition metal elements (seen for all configurations from d0 to d10) and also is often met for complexes of metal ions from s and p blocks of the Periodic Table. Of the two limiting shapes (Figure 4.17) the octahedral geometry is by far the most common though a few examples of the other limiting shape trigonal prismatic, exist. Because the six donor atoms come into closer contact in the trigonal prismatic than in the octahedral geometry trigonal prismatic is predicted to be less stable. However many structures show distortion that places them as intermediate between ideal octahedral and the ideal trigonal prismatic form. This distortion is best viewed in terms of the orientations of two opposite triangular faces of sets of three donors. For octahedral (which is also a special case of trigonal antiprismatic where edge and face length uniformity applies), the ‘top’ face is twisted around 60◦ versus the ‘bottom’ face; for trigonal prismatic, the two faces exactly superimpose (the twist angle has been reduced to 0◦). Intermediate or distorted structures show a twist angle of between 60◦ and 0◦ (Figure 4.17). The trigonal prismatic geometry is usually enforced by the ligand. Simple ligands almost exclusively form octahedral complexes, as do the vast majority of polydentate ligands. Of complexes of monodentate ligands, [Re(CH₃)₆] [Zr(CH₃)₆]₂− and [Hf(CH₃)₆]₂− (which are d0 or d1 systems) are three of a very few with simple-bonded ligands that display trigonal prismatic geometry. A fairly rigid chelate with a short separation between the two donor atoms (the so-called ‘bite’ of the ligand, the preferred separation of donors from each

Figure 4.17

Six-coordinate geometries. Views of the common octahedral and rare trigonal prismatic shapes with (a), (d) all faces defined; with the trigonal faces carrying sets of donor groups defined through views   perpendicular into the face (b) (e); and side on showing the ‘sandwich’ character of the L₃ML₃ sets (c), (f). Note how the angle between the triangular faces projected onto a common plane varies from 60◦ in the octahedral case to 0◦ in the trigonal prismatic case. Twisting of one octahedral face (1) with the other fixed in position through an intermediate (2) to the trigonal prismatic geometry (3) provides  a mechanism for interconversion without bond breaking.

other, influenced by their ligand framework) may also direct the shape to trigonal prismatic. Transformation between the two limiting geometries of octahedral and trigonal prismatic can occur simply by rotation of one triangular face relative to the other, as illustrated in Figure 4.17.

The dominance of octahedral geometry may be assigned to a number of factors: this arrangement is favoured by the amended VSEPR concept, is an ideal shape for minimizing steric clashing between donors, promotes good metal–ligand orbital overlap, and leads to favourable ligand field stabilization energies. Distortions from pure octahedral geometry can arise from twisting of faces, as discussed above and this effect is most likely met with chelate ligands, where the ‘bite’ of the chelate donor groups may be satisfied by a twisting distortion. Most chelates do not lead to this outcome, and octahedral predominates except that chelates with non-ideal ‘bite’ are those that often show some distortion away from ideal

Figure 4.18 CF3 L Examples of six-coordinate trigonal prismatic geometry.

octahedral. In the extreme, this may take the shape across to trigonal prismatic; fairly rigid chelates like dithiolates (−S (R)C=C(R) S−) may direct this outcome, as found in the trigonal prismatic Re (VI) complex [Re(S(CF₃) C=C(CF₃) S)3]. The tungsten (0) complex of a chelate with P-donors (Figure 4.18) also is trigonal prismatic.

Another way distortions occur is through the attachment of a mixture of ligands, since each donor type has a preferred bond distance. For example, although all bonds are equiva lent in [Cr(OH₂)₆]³⁺ when one is substituted to form [CrCl(OH₂)₅]^₂+theCr Cl distance is clearly different from the Cr-O distances; moreover, even the O-donor opposite the Cl ion is slightly different in distance from the chromium (III) ion than the remainder O-donors. Differences in steric bulk and electrostatic repulsion can add to these distortions; for cis [CrCl₂(OH₂)₄] +the Cl—Cr-Cl− angle is opened out compared with the H₂O Cr OH₂ angles, leading to a distorted octahedron. In reality it is rare to find a complex that adopts perfect octahedral shape, since we are commonly dealing with ligands or sets of ligands with different donors. These observations, of course, apply to any basic stereochemistry.

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)

Five Coordination (ML5)

Four Coordination (ML4)

Three Coordination (ML3)

Two Coordination (ML2)

Complex Lifetimes– Together Forever

?Preferences– Do You Like What I Like