Five-coordinated complexes Intermediates

Five-coordinate complexes are far more common than was once supposed and are now known for all configurations from d to d. Two limiting stereochemistries may be distinguished (Fig. 19.4). One of the first authenticated examples of 5-coordination was [VO(acac)2] which has the square-pyramidal 4 structure with the =0 occupying the unique apical site. However, many of the complexes with this coordination number have structures intermediate between the... 

The formation of the complexes shown in Scheme 14 and Eq. (5) has been rationalized according to Scheme 15. Thus, it has been proposed that the insertion of the Os—H bond of OsHCl(CO)(P Pr3)2 into the carbon-carbon triple bond of the alkynol initially gives five-coordinate (E )-alkenyl intermediates, which subsequently isomerize into the Os CH=CHC(OH)R1R2 derivatives. The key to this isomerization is probably the fact that the five-coordinated ( )-alkenyl intermediates are 16-electron species, while the Os CH=CHC(OH)R1R2 derivatives are... 

Six-coordinate complexes are expected to be distorted from pure octahedral symmetry by the Jahn-Teller eEFect and this distortion is generally observed (Chapter Ml. A number of five-coordinate complexes are known, both square pyramidal and trigonal bipyramidal. Four-coordination is exemplified by square planar and tetrahedral species as well as intermediate configurations. 

Addition of triphenylphosphine or t-butylisocyanide to 101 affords the analogous five-coordinate complexes 102b and 102c. These species represent rare examples of stable five-coordinate ds-dialkyl complexes of Ni(II), although stable trigonal-bipyramidal trans-dialkyls of nickel are well known (176-178), and they provide a structural model for a putative square-pyramidal intermediate in reductive elimination reactions of ds-dialkyl... 

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... 

Kinetically inert square-planar complexes are found by d8 low-spin ions, particularly Pt2+. Ligand substitution is associative and correlated with the ease of forming a five coordinate transition state (or intermediate). Substitution is much faster with Ni2+ where five-coordinate complexes such as [Ni(CN)5]3- are more stable than for Pt. For a given metal, the rate of substitution is controlled by ... 

Five-Coordinate Complexes. Both high-spin (three unpaired electrons) and low-spin (one unpaired electron) configurations are found for both trigonal-bipyra-midal and square pyramidal as well as for intermediate configurations. The following configurations are for the four spin-structure combinations ... 

Pt is almost always 16-electron and sqnare planar, as expected for a d metal, althongh rare five-coordinate complexes are sometimes formed. This is the most stable oxidation state. Substitntion reactions nsnally go by an associative pathway via an 18-electron five-coordinate intermediate. A high trans effect ligand snch as thionrea (tu) favors snbstitntion trans to itself by its preference for the equatorial site in the intermediate (see equation 2). 

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