Three-coordinate complexes

The structures and conformations of several cationic complexes [PtH(tetraL)] have been examined by NMR spectroscopic and X-ray structural studies (tetraL = Ph2PC2H4PPhC2H4PPhC2H4PPh2). Both meso and chiral versions of the ligand were investigated (dependent on the relative ligand conformations at the midchain phosphorus atoms). A variety of structural types were identified, some of which can interconvert by an intramolecular pseudorotation type of fluxionality. 

Protonation reactions of either [PtEt2(biL)] or [Pt(C2H4)(biL)] (biL = Bu2P(CH2) PBu25 n = l or 3) afford cations which are characterized by NMR spectroscopy as either ethene-hydride complexes or agostic bonded 

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Other iron-imido complexes have also been reported. Holland and coworkers reported the synthesis of the imidoiron(III) complex [L FeNAd] [40, 41]. This imidoiron(III) complex has not been isolated and was found to convert to a purple high-spin iron(III) complex. It has an S = 3/2 ground state from EPR measurement. Based on the results of QM/MM computations, [L EeNAd] is a three-coordinated complex with an Fe-N distance of 1.68 A and has a nearly linear Fe=N-C unit with Fe-N-C angle of 174.1°. Chirik and coworkers made use of liable ligands to prepare iron-imido complexes by treatment of C PDI)-Fe(N2)2 ( PDI = (2,6- Pr2CgH3N = CMe)2C5H3N) with a series of aryl azides [47]. 

Although in most of the above reports structural information is based on X-ray data, Au Mossbauer spectroscopy has also been successfully employed for the investigation of two-, three-, and four-coordination in gold(I) complexes.2551 An increase in coordination numbers leads to a decrease in the isomer shift (IS) by 1-2mm s 1 (three-coordination) or 2-4mm s 1 (four-coordination) relative to two-coordination. For the same ligands, the quadrupole splitting (QS) for three-coordinated complexes is expected to be very similar to that for the two-coordinate derivatives, while that for complexes with Td symmetry should be zero. 

A few comments on substitutional reactivity of Cr[CH(SiMe3)2]3 are buried within a 150 page review of three-coordinate complexes (92). 

Cummins, Christopher C., Three-Coordinate Complexes of Hard 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. 

Figure 6.3 Illustration of the three-coordinate complex [Co N(SiMe3)2 3]. Cobalt and nitrogen atoms are shown as black spheres, silicon atoms are grey and carbon atoms are given in white shadow. The Co-N bond length is 1.87A...

As mentioned above, the term of the rate law shown in Eq. 13.10 could also arise from dissociation (0) of X to give a three-coordinate complex which then reacts with Y. 

The spherically symmetric dll) configuration affords no LFSE, so the preferred coordination is determined by other factors. For Cu(l) the preferred coordination appears to be linear ( p), two-coordination, although three-coordinate complexes are known as well as several tetrahedral complexes. Zinc(Il) is typically either tetrahedral (e.g., [ZnCl4J2-) or octahedral (e.g., [Zn(H20)6]2 ), but both trigonal bipyramidal and... 

Table 6 Copper(I) Mononuclear Three-coordinate Complexes...

Copper and nickel complexes of the tridentate l-(2-carboxyphenyl)-3,5-diphenyl- (169 X = C02 R = R = Ph) and 1-(2-hydroxyphenyl)-3,5-diphenyl-(169 X = 0 R = R = Ph) formazans were prepared118 by the interaction of the formazan and the appropriate metal acetate in alcohol and were assigned the three-coordinate structures (170 X = O, C02 R = R = Ph M = Ni, Cu) since the diamagnetic nickel complexes were found to be unimolecular in benzene solution. Treatment of the nickel complex (170 X = O, R = R = Ph M = Ni) with pyridine gave a violet crystalline adduct which was assigned the four-coordinate structure (171 X = O R = R = Ph M = Ni). A product similar to the latter could not be obtained from the nickel complex of l-(2-carboxyphenyl)-3,5-diphenylformazan but nickel complexes of this type were obtained from both l-(2-hydroxyphenyl)- (169 X = O, R = CN R = Ph) and l-(2-carboxyphenyl)- (169 X = C02 R = CN R = Ph) 3-cyano-5-phenylformazans. In all three cases a considerable shade change occurred on going from the three-coordinate complex to the pyridine adduct. 

Thermolysis of these complexes at 9°C produced ethylene, cyclobutane, and butenes. The ratio of the gaseous products was found to be a function of the coordination number of the complex, or intermediate. Thus three coordinate complexes favoured butene formation, while four coordinate complexes favoured reductive elimination to form cyclobutane, and five coordinate complexes produced ethylene as shown in Scheme 25.83... 

Gosser, L.W. and Tolman, C.A. (1970) New three-coordinate complex of nickel (0). Tris(tri-o-tolyl phosphite)nickel. Inorg. Chem., 9, 2350. 

With the help of DFT calculations the stability of the three-coordinate complexes g was recently confirmed, but no evidence has yet been found for the five-coordinate intermediates h [14,15]. Instead, Goo (Sen et al. disclosed the linear complex i as an alternative structure for the initial intermediate [15]. It offers a valid explanation for Jutand s findings and reinstates the original four-membered intermediates in the catalytic cycle, albeit starting from anionic precursors (Scheme 3, bottom). 

The related three-coordinate complexes Mo[ CpRMo(CO)3)2Pb(thf)] were obtained as green crystalline solids in fairly good yield. Changing the reaction solvent from THF to a hydrocarbon resulted in the formation of an unusual bridging carbonyl dimeric Mo-lead complex (Cp Mo(CO)3)PbCp Mo(CO)2(/z-CO) 2, HS4 517. 

Total cLOrbital Stabilization Energies [ ([Pg.131]

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