Four-coordinate species

The products obtained from the Pt(PR3)2X2-CNCH3 reactions were dependent on the nature of the platinum species. Five-coordinate adducts, [Pt(PR3)2(CNCH3)2X]X, were isolated for the iodo and bromo complexes (R=Ph), although the latter was unstable and slowly lost isocyanide. The observation of five-coordination here is somewhat unusual, but since this report, it was also observed in a different situation (85), mentioned above. The more common observation was the isolation of four-coordinate species, implying the low stability of most five-coordinate complexes. Data on these reactions are summarized below [Eqs. (33, 34)]. 

Square-planar zinc compounds predominate with these ligand types as would be predicted. This is in contrast to the prevalence of tetrahedral or distorted tetrahedral geometries for four-coordinate species that have been discussed thus far. Zinc porphyrin complexes are frequently used as building blocks in the formation of supramolecular structures. Zinc porphyrins can also act as electron donors and antenna in the formation of photoexcited states. Although the coordination of zinc to the porphyrin shows little variation, the properties of the zinc-coordinated compounds are extremely important and form the most extensively structurally characterized multidentate ligand class in the CSD. The examples presented here reflect only a fraction of these compounds but have been selected as recent and representative examples. Expanded ring porphyrins have also... 

The equilibrium ground-state spectra of the parent six-coordinate and deoxy five-(or four-)coordinate species were obtained using a Tungsten lamp placed before the I /I beam splitter. Earlier work in our laboratory (3,4) has established that the experimental spectra of transient protoproducts can be resolved as they sequentially evolve during the dissociation process. At relatively long delays > 20 psec after excitation, the spectrum of the stable photoproduct... 

Nickel(II) complexes display a variety of equilibria which involve spin state changes. Planar four-coordinate complexes are invariably diamagnetic. These can undergo an intramolecular isomerization to paramagnetic tetrahedral four-coordinate species. Alternatively, the planar complexes can coordinate additional ligands to form five- and six-coordinate paramagnetic complexes. The additional ligand molecules can be Lewis bases in solution, or solvent molecules, or, in par-... 

Only four-coordinate species are known for Ni11 and Pd11 corroles.239... 

It is of interest to compare the porphyrin and corrin macrocycles, and to consider the reactivities of porphyrin- and corrin-bound metal ions. These may be related to the size of the cavity. The corrin macrocycle provides a smaller cavity, into which cobalt fits tightly in the B12 coenzymes. Thus the cobalt does not move relative to the corrin plane, and remains coplanar with the four nitrogen donor atoms. The cobalt remains low-spin, and reactivity is controlled by the loss of the axial ligands, giving five- and four-coordinate species. 

Numerous experimental observations of the increased reactivity of pentacoordinate silicon over tetracoordinate silicon have been reported. Corriu and coworkers have found that the relative reactivity of PbMeSiI 3 and PhMeSiF2 towards t-BuMgBr was >1000 155. The pentacoordinate species 29 reacts with alcohols and acids to give mono- or disubstitution products whereas the corresponding four-coordinate species 30 does not react at all56. 

The introduction of a phenyl substituent in the ortho position of the aryl-ether unit in 36 supports the formation of the catalytically active four-coordinate species derived from 34 leading to the increase in activity by two orders of magnitude, an effect which has previously been observed for derivatives of the achiral catalyst [76,77]. 

The criteria for relative stability of five- and four-coordinate species are by no means fully established. Substitution reactions of square species, which are often... 

---