Platinum planar

EC, electrode cells EC, filling chambers SP, silicon packings C, rectangular capillary M, microporous PTFE membranes E, platinum planar electrodes S, stoppers [68. ... 

P, Petri dish TS, PTFE supports PP, polymerized packing (Lukopren ) G, glass seal, E, platinum planar electrodes TW, thermostated water [68. ... 

Compound [PtCl( Bu2PCMe2CH2)2l reacts with pyrazole or 3,5-dimethyl-pyrazole in the presence of sodium hydroxide to form 242 (R = H, Me) [84ICA (82)L9]. The chelate ring is not planar in this case, and the trans strucmre of the pyrazolate derivative was demonstrated. Tlie four-coordinated platinum atoms are characterized by a distorted square-planar coordination. 

The introduction of the sample into the adsorbent layer is a critical process in HPTLC. For most quantitative work a platinum-iridium capillary of fixed volume (100 or 200 nL), sealed into a glass support capillary of larger bore, provides a convenient spotting device. The capillary tip is polished to provide a smooth, planar surface of small area (ca 0.05 mm2), which when used with a mechanical applicator minimises damage to the surface of the plate spotting by manual procedures invariably damages the surface. 

However, it seems that these are best viewed as platinum(II) species too, so that two-electron metal-to-ligand transfer has been effected. The structures of Pt(PPh3)2Z (Z = r]2-02, t 2-C3H4, t]2-CS2) (Figure 3.16) all involve square planar coordination as expected for platinum(II) rather than the tetrahedral 4-coordination anticipated for platinum(O). 

The adoption of a planar structure in these adducts, rather than the sterically more favourable tetrahedral one, is in keeping with a platinum(II) oxidation state. The side-on bonding of the 02 molecule is believed to involve two components, as in Zeise s salt (Figure 3.18). 

There is significant metal-metal bonding in the platinum compound, whose geometry involves a square of platinum atoms another important difference is that the coordination geometry is square planar in palladium acetate but octahedral in the platinum analogue. Different oligomers exist in solution, broken down by adduct formation. Palladium(II) acetate may be obtained as brown crystals from the following reaction [65] ... 

With their preference for square planar coordination, palladium(II) and platinum(II) are well suited to binding to porphyrins and related N4 donor macrocycles. Therefore, Pd(octaethylporphyrin) is readily synthesized starting from the labile PhCN complex (like the platinum analogue) [92]... 

The platinum complex is square planar, while the palladium dimer also has planar 4-coordination (for other examples of mercaptide bridges see section 3.8.3) [116]. 

The 4 1 complex has square planar coordination of platinum (Pt-S 2.317— 2.321 A) similar bond lengths are found in the corresponding complex with 1,4-thioxane [120]. Complexes with thiourea are important in Kurnakov s test (section 3.8.2) Pdtu4Cl2 has square planar coordination (Pd—S 2.33 A). 

Square planar complexes of palladium(II) and platinum(II) readily undergo ligand substitution reactions. Those of palladium have been studied less but appear to behave similarly to platinum complexes, though around five orders of magnitude faster (ascribable to the relative weakness of the bonds to palladium). 

Since n bonding is believed to be more important in low oxidation states, as d orbitals contract with increasing oxidation state leading to poorer dw-pw overlap, this would not be expected on the basis of a 7r-bonding mechanism. Similarly, one can compare /(Pt-P) for pairs of isomers in the +2 and +4 states in a planar platinum(II) complex, the platinum 6s orbital is shared by four ligands whereas in an octahedral platinum(IV) complex it is shared by six ligands. Therefore, the 6s character is expected to be only 2/3 as much in the platinum(IV) complexes, correlating well with the 7(Pt-P) values, which can be taken to be a measure of the a-character in the bond. 

Oxidation of [Pt(C6Cl5)4]2- yields the unusual paramagnetic organo-metallic [Pt(C6Cl5)]4 with square planar coordination of platinum... 

Like palladium(II) and platinum(II), gold(III) has the d8 electronic configuration and is, therefore, expected to form square planar complexes. The d-orbital sequence for complexes like AuC14 is dx2 yi dxy > dvz, dxz > dzi in practice in a complex, most of these will have some ligand character. 

Application of the principle of microscopic reversibility can be used to eliminate a mechanism suggested at one time for the nucleophilic substitution reactions of square-planar platinum(II) complexes. For the sake of specificity, we take PtCl - as a typical... 

As already mentioned, complexes of chromium(iii), cobalt(iii), rhodium(iii) and iridium(iii) are particularly inert, with substitution reactions often taking many hours or days under relatively forcing conditions. The majority of kinetic studies on the reactions of transition-metal complexes have been performed on complexes of these metal ions. This is for two reasons. Firstly, the rates of reactions are comparable to those in organic chemistry, and the techniques which have been developed for the investigation of such reactions are readily available and appropriate. The time scales of minutes to days are compatible with relatively slow spectroscopic techniques. The second reason is associated with the kinetic inertness of the products. If the products are non-labile, valuable stereochemical information about the course of the substitution reaction may be obtained. Much is known about the stereochemistry of ligand substitution reactions of cobalt(iii) complexes, from which certain inferences about the nature of the intermediates or transition states involved may be drawn. This is also the case for substitution reactions of square-planar complexes of platinum(ii), where study has led to the development of rules to predict the stereochemical course of reactions at this centre. 

Substantially more work has been done on reactions of square-planar nickel, palladium, and platinum alkyl and aryl complexes with isocyanides. A communication by Otsuka et al. (108) described the initial work in this area. These workers carried out oxidative addition reactions with Ni(CNBu )4 and with [Pd(CNBu )2] (. In a reaction of the latter compound with methyl iodide the complex, Pd(CNBu )2(CH3)I, stable as a solid but unstable in solution, was obtained. This complex when dissolved in toluene proceeds through an intermediate believed to be dimeric, which then reacts with an additional ligand L (CNBu or PPh3) to give PdL(CNBu )- C(CH3)=NBu I [Eq. (7)]. 

C20-0004. Platinum forms a large collection of square planar complexes. Draw ball-and-stick models similar to those in Example for the cis and trans Isomers of [Pt (NH3)2 CI2 ]. 

The Pd lT) and Pt ll) complexes yilB dtc) HI, 46) are diamagnetic and have a square planar geometry 112,113) like the nickel analogues. In CH2CI2 they can be oxidised irreversibly on a rotating platinum electrode, the Pd compound at a higher potential than the Pt compound (1.21 and 0.92 V vs s.c.e., respectively) 150). 

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