Five-coordinate molecules rearrangements

Many five-coordinate molecules exhibit very rapid intramolecular rearrangements, leading to all five ligands being equivalent over the NMR timescale, with the energy barriers to interconversion... 

Many five-coordinate molecules exhibit very rapid intramolecular rearrangements, leading to aU five ligands being eqnivalent over the NMR timescale. For example, the F NMR spectmm of PFs shows aU finorine atoms eqnivalent over the temperatnre range 60 to — 197°C. Proton NMR stndies on SbMes in carbon disnlfide down to abont —100 °C also show the presence of only one type of methyl gronp. The C NMR spectra of [Fe(CO)s] and [Fe(CNBu)s]... 

Hydrogenation proceeds by hydride rearrangement of 17 to a five-coordinated ethyl-rhodium complex, 18. This complex regains a ligand molecule to replace the one lost previously, thereby giving the six-coordinated complex, 19 ... 

Dinucleating carboxylates support diiron(II) complexes that bind O2 rapidly in a simple bimolecular process (first order in the complex and first order in O2).92 The activation parameters estimated for the oxygenation of the XDK-Im complex, approximately A/T = 16 kJ/mol and AS = —120 J/(molK) are in accord with the values obtained for other diiron(II) complexes and correspond to a low-barrier O2 coordination at a vacant iron(II) center. A mechanism proposed for the oxygenation reaction includes the attack of the O2 molecule on the unsaturated (five-coordinate) iron(II) center concomitant with the carboxylate shift at this center followed by the coordination to the second, six-coordinate iron(II) center and additional ligand rearrangement (Figure 4.26). 

Figure 6.18 illustrates the volume profiles for the reactions of NO with the metMl HiO) and for the analogous reaction with (TPPS)Fe" (H20)2. Dissociation of a coordinated water molecule from an octahedral metal center is expected to be accompanied by a maximum volume increase of 13 cm mol [97]. The larger value of AV reported here for metMb suggests that the protein may undergo some structural rearrangement during the formation of the five-coordinate metMb. ... 

Molecular motors or machines are inspired by biological molecules such as myosin which uses the chemical energy from hydrolysis of adenosine triphosphate to drive the linear push-pull motion of muscle. The different coordination demands of Cu and Cu are the basis of electro-chemically induced molecular motion in a pseudorotaxane complex of copper. As shown in Scheme 2, Cu 4, the stable, four-coordinate form is oxidized to unstable Cu°4, which rearranges to the stable five-coordinate form by sliding along the ligand. Reduction of the stable Cir s... 

The suggested mechanism is depicted in Scheme 1. However, the kinetic data observed taken alone do not distinguish between insertion from a rearranged four-coordinate intermediate (Scheme 1) and an associative mechanism involving a five-coordinate intermediate of the type [PtH(ethylene)(PEts)a(Z)], where Z is a coordinated solvent molecule, alkene, or counterion, a mechanism which may become competitive for poor leaving groups trans to hydride ligand. ... 

Certain cis-trans isomerizations of square planar [notably Pd(II) or Pt(II)] complexes occur spontaneously but most are catalyzed by donor molecules, which may be free ligands added or generated in solution or coordinating solvents. Two of the mechanisms that have been considered for the catalyzed reaction are consecutive displacements with four-coordinate intermediates or intramolecular rearrangements of five-coordinate transient species. The exchange of methyl groups in 1 is strongly catalyzed... 

---