Five-coordinate molecules

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... 

X-ray diffraction studies on hydrated triorganotin compounds H2O — SnRsX have provided several examples of water engaged in inner-sphere coordination to tin in five-coordinate molecules. For inner-sphere coordination, the complexes tend to pack in arrangements in which the number of hydrogen bonding interactions involving the coordinated water molecule is maximized. For example, o-phenanthroline monohydrate, which chelates to a plethora of transition metals, forms with triphenyltin chloride an outer-sphere coordination complex in which the coordinated water molecule [O — Sn = 2.42 A]... 

In general, the isomerizations of molecules which have skeletons with s positions can be described by permutations of the symmetrical group S,. Thus, the formal mechanistic alternatives for the interconversions of permutational isomers of five-coordinated molecules are obtained by partitioning the group S5 in correspondence to the partitions of the number five into its distinct seven classes of conjugate elements ... 

A triamidoamine ligand ((NjN) " = ((Mc3SiNCH2CH2)3 N) ) has facilitated the preparation of a new class of chromium(IV) organometalhcs. Alkyls (N3N)CrR (R = Me, n-Bu) as well as the hydride (N3N)CrH have been characterized by X-ray diffraction. The five-coordinate molecules adopt high-spin d configuration. 

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]... 

Phosphorus peniafluoride ( >3/,). A Lewis structure for the PFj molecule requires ten electrons in the valence shell of the phosphorus atom and the use of 3s, 3p, and l>d orbitals and five cr bonds. It is impossible to form five bonds in three dimensions such that they are all equidistant from one another, but the trigonal bipyramidal (Fig. 6.1e) and square pyramidal arrangements tend to minimize repulsions. Almost every five-coordinate molecule (coordination compounds excepted) which has been carefully investigated has been found to have a trigonal bipyramidal structure. The structure of the PF5 molecule is shown in Fig. 6.1e sp d hybrid). The bonds are of two types axial, the linear F—P—F system and equatorial, the three P—F bonds forming a trigonal plane. 

Main group five-coordinate molecules are found cither as trigonal bipyramidal molecules (c.g., PFs) or as square pyramidal species (c.g., BrFs). Geometrically they are quite close as. shown in 14.20, a projection down the fourfold axis of the... 

In a five coordinate molecule or complex (5.5) it is possible to interchange the axial (A) and equatorial (E) ligands without dissociation via the square-pyramidal intermediate (5.6). 

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