Five-coordinate compounds

Despite the fact that PtL3X2 and PtX species have an 18-electron configuration, 5-coordinate palladium(II) and platinum(II) compounds are rare. One of the first examples to be established was Pt(SnCl3)5  

Some 5-coordinate phosphite complexes (fluxional at room temperature) exist 

steric effects prevent more than four bulky phosphites coordinating [135]. 

Ficwe 3.80 Phosphine U nds forming the S-coordinate palladium and platinum complexes. 

Complexes of bulky substituted phenanthrolines [Pt(N-N)LX2] (L, X both monodentate N-N, e.g. 2,9-dimethyl-l,10-phenanthroline) can be 5-coordinate tbp when a good 7r-acceptor (e.g. C2H4) is present or 4-coordinate with monodentate phenanthrolines. Hartree-Fock calculations indicate that the TT-acceptors reduce the electron density at platinum so that the metal can accept charge from another donor. Species of this kind may be involved in alkene hydrogenation [138]. 

In the 1920s, the Russian chemist Il ya Il ich Chernyaev systematized reactions of complexes of several metals, particularly platinum(II and IV), by noting that a ligand bound to a metal ion influenced the ease of replacement of the group trans to it in the complex [139]. 

195Pt and 119Sn NMR data show Pt(SnCl3)5 to be non-rigid (on the NMR timescale) down to 183 K, owing to an intramolecular process, possibly a Berry twist mechanism [132]. 

Finally, an application of cross-polarization magic-angle spinning to NMR 

A theoretical explanation of the kinetic trans-effect at square-planar molecules in terms of the angular deformability of the bond in the trans-activating ligand has been presented.The ability of these ligands to leave the plane at low energy cost determines the ease by which a particular 5-coordinate intermediate will form. 

The complex [Pt(dmso)4](S03CF3)2 has two cis S-bonded ligands and two O-bonded. Comparison of its crystal structure with that of its palladium analog reveals significantly longer Pd—S bonds. The crystal structure of the rhodium complex (13) confirms the expected greater trans-influence of S over 

The normal trans-directing influence fails in the reactions of cis- and trans-[PtCl2(NH3)2] with dmso. One chloride is replaced by dmso in both isomers. As the process is rapid, and the products have a different activity toward biological chemicals, the authors warn against the use of dmso as solvent for metal-containing anticancer drugs. 

The relationship of these compounds to the reaction intermediates of associative ligand exchange processes of 4-coordinate species makes their formations and reactions a rewarding area for study. This time there has been a great deal of structural activity in this field, and the results relate directly to many fundamental mechanistic questions. 

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Spot tests, 1, 552 Square antiprisms dodecahedra, cubes and, 1, 84 eight-coordinate compounds, 1,83 repulsion energy coefficients, 1, 33, 34 Square planar complexes, 1,191, 204 structure, 1, 37 Square pyramids five-coordinate compounds, 1,39 repulsion energy coefficients. 1,34 Squares... 

In a crystal-field picture, the electronic structure of iron in the five-coordinate compounds is usually best represented by a (d yf idyz, 4cz) ( zO configuration [66, 70], as convincingly borne out by spin-unrestricted DFT calculations on the Jager compound 20 [68]. The intermediate spin configuration with an empty d 2 yi orbital in the CF model, however, has a vanishing valence contribution to the... 

C-C couplings with aUcynes. An unprecedented coupling of this type was found in the reaction of the Ir(I) hydroxo-allenylidenes 71 with excess of HC=CR (R = Ph, C02Me) to afford, under remarkably mild conditions (r.t.), the novel five-coordinate compounds 72 (Scheme 24). The proposed mechanism involves an initial HO /R C=C ligand exchange followed by the oxidative addition of a... 

The structures of five-coordinate compounds can be classified theoretically into six different structural types, progressing from those containing five... 

Fig. 3a-c. Examples of five-coordinate compounds with five separate ligands... 

There are two basic structures possible for five coordinate compounds incorporating bidentate chelates either a compound with three free ligands... 

Another example of a Bill compound is offered by the ligand VAPOL ( vaulted bisphenanthrol), which combines with AICI3 to form a three-coordinate complex, with no dative interactions. This compound will catalyze Diels-Alder reactions [64]. A five-coordinate compound, with two dative carbonyl bonds attached to the aluminum, has been postulated as the active intermediate in the catalytic reaction. 

Each class of five-coordinate compound was further subdivided into the possible Types of structures that may be envisioned to occur. The majority of the compounds fell into a very few Types in each Class, and these will be noted separately below. These Types should provide a starting point for future reviews that can focus on the details of these compounds. Of key importance are the Types that had no members whatsoever. These provide clear targets for future synthetic efforts. 

Various five-coordinate compounds are known for the configuration d of the oxovanadium(IV) and titanium(III). The complexes of the oxo-vanadium can have structures (more or less distorted) referable to the trigonal bip5n amid or more commonly to the square p n amid. 

Limiting Geometries Although five-coordinate compounds are still less common than those of either coordi-... 

As noted above in Section 7.3.1.1, the five-coordinate compound [Be(Cl)(12-crown-4)]2+ has been prepared.178 Quantum mechanical calculations were undertaken on the hypothetical water and ammonia analogues, where the chloro ligand is replaced. It was concluded on the basis of the calculations that both water and ammonia exchange on [Be(H20)(12-crown-4)]2+ and [Be(NH3)(12-crown-4)]2 +, respectively, by an Ia mechanism.179... 

Unlike with nitrogen, the five-coordinate compounds Ph5E are known. The P and As compounds have the normal trigonal bipyramidal geometry but Ph5Sb is unexpectedly square pyramidal (2). 

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