Transition series metals

Metal Preferences. LVC s are formed mainly by transition metals to the right in the periodic table (especially elements in Group 8). This is in part due to the availability of d electrons that can be used in back-donation to the 7r-accepting ligands. Moreover, the formation of LVC s is not particularly "row-sensitive" by which I mean that the first-transition-series metals, Fe, Co and Ni, tend to form most of the same cluster compounds as their congeners, Ru, Rh, Pd and Os, Ir, Pt. 

The discussion above has been directed principally to thermally induced spin transitions, but other physical perturbations can either initiate or modify a spin transition. The effect of a change in the external pressure has been widely studied and is treated in detail in Chap. 22. The normal effect of an increase in pressure is to stabilise the low spin state, i.e. to increase the transition temperature. This can be understood in terms of the volume reduction which accompanies the high spin—dow spin change, arising primarily from the shorter metal-donor atom distances in the low spin form. An increase in pressure effectively increases the separation between the zero point energies of the low spin and high spin states by the work term PAV. The application of pressure can in fact induce a transition in a HS system for which a thermal transition does not occur. This applies in complex systems, e.g. in [Fe (phen)2Cl2] [158] and also in the simple binary compounds iron(II) oxide [159] and iron(II) sulfide [160]. Transitions such as those in these simple binary systems can be expected in minerals of iron and other first transition series metals in the deep mantle and core of the earth. 

Most commonly, metal ions M2+ and M3+ (M = a first transition series metal), Li+, Na+, Mg2+, Al3+, Ga3+, In3+, Tl3+, and Sn2+ form octahedral six-coordinate complexes. Linear two coordination is associated with univalent ions of the coinage metal (Cu, Ag, Au), as in Ag(NH3)2+ or AuCL Three and five coordination are not frequently encountered, since close-packing considerations tell us that tetrahedral or octahedral complex formation will normally be favored over five coordination, while three coordination requires an extraordinarily small radius ratio (Section 4.5). Coordination numbers higher than six are found among the larger transition metal ions [i.e., those at the left of the second and third transition series, as exemplified by TaFy2- and Mo(CN)g4 ] and in the lanthanides and actinides [e.g., Nd(H20)93+ as well as UC Fs3- which contains the linear uranyl unit 0=U=02+ and five fluoride ligands coordinated around the uranium(VI) in an equatorial plane]. For most of the metal complexes discussed in this book a coordination number of six may be assumed. 

The metals which form this type of complex by reaction with dioxygen are mainly the later second and third row transition series metals, notably Ru°, Os°, Rh1, Ir1, Pt° and Pd°. It is not surprising to see metals noted for their ability to undergo oxidative addition reactions amongst those which form this type of mononuclear dioxygen complex as a formal two-electron reduction of dioxygen is required for complex formation. The other metals known to form mononuclear peroxo-type complexes with dioxygen are Ni°, Co1 and one example of Co . 

We first treat those complexes that have only a metal-ligand bonding. Examples of this type of complexes include [M(H20)6] + and [M(NH3)6] +, where M + is a first transition series metal ion and a typical n value is either 2 or 3. A coordinate system for such a complex is shown in Fig. 8.8.1. The representation generated by the six ligand orbitals is... 

Complexes formed by heterocyclic amines with halides and pseudohalides of dipositive first-transition-series metal ions are usually prepared by the reaction between a metallic salt, a soluble thiocyanate, and pyridine in aqueous solution. Although the number of coordinated pyridine molecules varies,... 

It will also be noticed that all the above catalysts contain second transition series metals. Generally, the slower reactions of the third transition series elements are not normally conducive to catalytic efficiency, although some very active iridium catalysts are now known. First transition series metals seldom form stable, lower oxidation state tertiary phosphine complexes. 

Complexes of the rare earth elements provide examples of case (1), many complexes of the first-transition series metals provide examples of case (2), and complexes with covalent bonds provide examples of case (3). Case (2) will be considered here. Excellent monographs cited above are available in which the theory is developed in detail. 

Table 2. Known pentafluorides of the second and third transition-series metals ...

The third-transition-series metal hexafluorides are excellent for this purpose in that they constitute a set, well graded in oxidizing power [7], yet dmost isodimensiond moreover, as gases they may be readily transported and applied, and their activity controlled, to some extent, by pressure regulation. 

Turner et al. (1981) and Byrne et al. (1988). Turner et al. (1981) used a database of stability constants for more than 500 metal complexes to calculate the inorganic speciation for 58 trace elements in model seawater at pH 8.2, 25 °C, and 1 atm. Byrne et al. (1988) extended this work by considering the influence of temperature and pH on speciation. The free hydrated divalent cation dominates the dissolved inorganic speciation of Zn(II) and the first transition series metals Mn(II), Co(II), and Ni(II). Strongly hydrolyzed trace metals include Be(II), Al(III), Fe(III),... 

Table 2.2 The fractional absorption (f) and the notional equivalent half-time (Tgg) of retention in the human body of some main group and transition series metals. The fi is the fraction of an orally administered element which passes from the gastrointestinal tract to the blood stream the notional Tgg assumes a constant rate of loss from a single compartment...

The bonding to transition metal ions has been described in the previous section as being predominantly that of dative covalent bonding whereby lone pairs of electrons are donated into vacant orbitals of the transition series metal ions. It was established by Werner that these bonds are directional in nature and have certain preferred angles subtended at the central metal ion. 

Electron Compounds containing Transition Series Metals. 

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