Block d: Transition Metals

The d-block transition metals which form a group of elements ten-wide and four-deep in the Periodic Table associated with filling of the five d orbitals, represent the classical metals of coordination chemistry and the ones on which there is significant and continuing focus. In particular, the lighter and usually more abundant or accessible elements of the first row of the d block are the centre of most attention. Whereas stable oxidation states of p-block elements correspond dominantly to empty or filled valence shells, the d-block elements characteristically exhibit stable oxidation states where the nd shell remains partly filled; it is this behaviour that plays an overarching role in the chemical and physical properties of this family of elements as covered in earlier chapters.

The ability of complexes of d-block metal ions to readily undergo oxidation or reduction involving one or several electrons is a key feature of their chemistry. Because many transition elements have this capacity to exist in a range of oxidation states they offer different chemistry even for the one element as a result of the differing d electrons present in the different oxidation states; each oxidation state needs to be considered separately. Lighter elements of the d block tend to prefer O-.N- or halide ion donor groups whereas heavier elements lean towards coordination by ligands featuring heavier p-block elements such as S- and P-donors. The diversity of oxidation states, shapes and donor groups met in transition metal chemistry makes it both fascinating and frustrating - it is not simple chemistry.

The d-block elements display perhaps the greatest variation of the various groups with the first series row (with a 4s3d valence shell) in particular distinct from the second (5s4d) and third (6s5d) series rows. The fully synthetic fourth (7s6d) row has little chemistry on which to report yet. Radii of heavier elements and ions are larger than those of the first series, although lanthanide contraction determines that radii of the 5d series differ little from radii of the 4d series, despite increased atomic number. Higher oxidation states are much more stable for 4d and 5d than for 3d compounds; some oxidation states have no analogues in the 3d series. The +II oxidation state is of relatively little importance in 4d, 5d series (except for Ru), but is of major importance in the 3d series. The +III oxidation state is dominant in the 3d series, but relatively unimportant in 4d, 5d (exceptions are for Rh, Ir, Ru and Re). Rather, +IV +V and +VI are more often met with 4d and 5d metals. The 4d, 5d rows are much more prone to metal-metal bonding than the 3d row and multiple metal-metal bonds are common (Figure 6.1). For 3d, M-M bonds are only common in metal carbonyls. Polynuclear (and cluster) compounds are more common for 4d. 5d than for 3d species. Magnetic properties differ, with heavier elements tending to form dominantly low-spin compounds. Higher coordination numbers (>6) are also more common for 4d, 5d compounds.

If we examine a few of the columns within the d block selectively, the chemistry behind these generalizations becomes a little clearer. It is in group 9 (cobalt rhodium iridium) that the greatest similarities exist so it is instructive to explore it first. Cobalt is a classical 3d element, with Co(II) and Co (III) the sole significant oxidation states (although Co(IV) Co(I) and Co(0) are known but rare), with a preference for N-, O- and halide donor atoms. Complexes dominantly feature octahedral stereochemistry, particularly for Co (III); four- and five-coordinate Co(III) are very rare and usually found only with bulky ligands, although reactivity of such complexes is high, and known Co(III) metalloenzymes which participate in fast reactions have five-coordination. Four-coordinate Co(II) compounds are known with halide and soft donor groups. Whereas Co(IV) is extremely rare, it is more common for Rh and Ir, with particularly stable Ir(IV) compounds such as [IrC6l2- existing albeit as a

Figure 6.1 Examples of metal-metal bonded complexes of the second and third row d-block elements molyb- denum and tungsten. The Mo-Mo bond is quite short, and this is an example of a compound with a multiple metal-metal bond; other examples display single metal-metal bonds.

powerful oxidant capable of oxidizing hydroxide ion in aqueous solution to oxygen. The M(III) oxidation state is common for all of Co, Rh and Ir, but the inertness of complexes increases down the column; typical reactivity trends for the complexes Co: Rh:Ir are ~1000: 50: 1. This reactivity trend is characteristic of the d block in general. It is for the M(II) state that differences are starkest; Co(II) is common and stable, whereas Rh(II) and Ir(II) form few stable monomeric complexes. However, in yet lower oxidation states we find that all three metals form similar and fairly common compounds. Despite some common behaviour, there are clear differences across their chemistries: for example polyhalo complexes such as [IrCl] and [RhCl₅(OH)] are stable, whereas no more than three halide ions, as in [CoCl₃(NH₃)₃] can be present for stability in Co(III); also hydride (H-) complexes of Rh and Ir are common with even the simple species [RhH(NH₃)₅]²⁺ formed, whereas Co does not form such species.

Group 7 (manganese. technetium. rhenium) is one with clearer differences between the chemistry of the lightest member and the two heavier members. Manganese is known in oxidation states all the way from Mn(I) to Mn (VII) with a changeover from preferred six- to four-coordinate complexes occurring around Mn(V). The Mn (II) oxidation state is common, with higher oxidation states progressively less common, although some very important compounds exist at higher oxidation states namely the simple oxide Mn1VO₂ and the powerful and popular oxidant MnO4. Technetium and rhenium have no analogue of Mn²⁺ aq and form very few M(II) species; indeed they have little cationic chemistry in any oxidation state. Unlike Mn they have an extensive chemistry in the M(IV) and M(V) oxidation states; the latter is the least common for Mn. The formation of clusters and M-M bonds is much more common for Te and Re than with Mn, and is a feature of the (II) and (IV) oxidation states for Tc and Re. Much of Tc. Re chemistry resembles more that of adjacent neighbours Mo. W than they do Mn, despite their different valence electron sets. Diversity is a key expectation of d-block chemistry.

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المزيد

الآخبار الصحية

اكتشاف هام قد يمهّد لعلاج جديد للسمنة

تغييرات بسيطة في نمط الحياة تقلل خطر أمراض القلب بنسبة كبيرة

ليس اللب فقط!.. قشرة وبذور المانغو تخفي فوائد صحية مذهلة

زيادة استهلاك الكالسيوم ومنتجات الألبان يحمينا من مشكلات صحية خطيرة

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الآخبار التكنلوجية

العلماء الروس يكتشفون طريقة تحسن فرص نجاح زراعة الأسنان

لحظة زمنية خاطفة في الدماغ تتحكم في التعلم والحركة

القمر لم يعد مجرد محطة.. خطوة جديدة تغيّر مستقبل البشرية

العلماء يكتشفون "نوعا جديد تماما" من الكواكب

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مواضيع ذات صلة

Block f : Inner Transition Metals (Lanthanoids and Actinoids)

Block p: Main Group Metals

= Block s : Alkali and Alkaline Earth Metals

A New Face - Oxidation-Reduction

A New Body - Stereochemical Change

Lability and Inertness in Octahedral Complexes

Kinetic Rate Constants

Thermodynamic and Kinetic Stability

Undergoing Change– Kinetic Stability

Overall Stability Constants

Factors Influencing Stability of Metal Complexes

Bedded Down– The rmodynamic Stability