Pair diamagnetic

B—These have completely filled shells or subshells—all the electrons are paired = diamagnetic. 

It behaves, hardly surprisingly, very like PhjC- cf. p. 300), existing out of solution as a colourless solid, but this latter is probably a polymer rather than a dimer as with Ph3C. The solid is dissociated in solution to about the same extent as the PhjC- dimer. The unpaired electrons in the biradical form (135) cannot interact with each other to form a wholly paired, diamagnetic sjwcies, as such interaction across both central benzene nuclei would necessitate m-quinoid forms that cannot exist the electrons are thus internally insulated from each other. Such internal insulation in biradicals may also arise through steric rather than electronic causes. Thus the species (136) exists in solution as a biradical to the extent of w 17 %, being in equilibrium with a polymer (like 135) ... 

Potassium ferrocyanide, K4[Fe(CN)6] 3H20 occurs as a trihydrate. The compound is a spin-paired diamagnetic complex in which the ferrocyanide anion constitutes the Fe2+ ion, octahedrally coordinated with six CN ions. It is a yellow monoclinic crystaUine sohd density 1.85 g/cm decomposes at 60°C and soluble in water but insoluble in alcohol and ether. 

In this absence of evidence for increased radical density, one could postulate the presence of spin-paired, diamagnetic, dianions in the reduced coal. However, using the chemistry of polycyclic aromatic hydrocarbons as a model, we find hat such dianions are neither expected to be statistically abundant (24, 25) nor, as good bases, to be inert to the reductive system on a time scale of 100 hours (26). 

Thorn, R. J. "Thermochemistry of the Electron-Paired, Diamagnetic State of Superconductivity," Argonne National Laboratory, Argonne, IL 60439, preprint. 

Square-planar complexes are often formed when a metal has 8 electrons in d orbitals, as is the case with Pt", Pd , Au , Rh and Ir . Such square complexes are always spin paired (diamagnetic). The reactions of Rh and IP are very fast and have been therefore much less studied. On the other hand, the mechanistic information collected in reactions of the platinum(II) complexes can be in most cases applied to the square-planar complexes of other metals. 

CIDNP involves the observation of diamagnetic products fonned from chemical reactions which have radical intemiediates. We first define the geminate radical pair (RP) as the two molecules which are bom in a radical reaction with a well defined phase relation (singlet or triplet) between their spins. Because the spin physics of the radical pair are a fiindamental part of any description of the origins of CIDNP, it is instmctive to begin with a discussion of the radical-pair spin Hamiltonian. The Hamiltonian can be used in conjunction with an appropriate basis set to obtain the energetics and populations of the RP spin states. A suitable Hamiltonian for a radical pair consisting of radicals 1 and 2 is shown in equation (B1.16.1) below [12]. 

Unlike nitrogen monoxide, nitrogen dioxide has properties more typical of an odd electron molecule. It is a coloured (brown), reactive gas which dimerises to the diamagnetic colourless gas dinitrogen tetroxide, N2O4. in which the odd electron is paired. The structure of dinitrogen tetroxide can be represented as a resonance hybrid of ... 

Hund s rule, like the Pauli exclusion principle, is based on experiment It is possible to determine the number of unpaired electrons in an atom. With solids, this is done by studying their behavior in a magnetic field. If there are unpaired electrons present the solid will be attracted into the field. Such a substance is said to be paramagnetic. If the atoms in the solid contain only paired electrons, it is slightly repelled by the field. Substances of this type are called diamagnetic. With gaseous atoms, the atomic spectrum can also be used to establish the presence and number of unpaired electrons. 

In the Fe(CN)64- ion shown at the left, Ao is so large that the six 3d electrons of the Fe2+ ion pair up in the lower energy orbitals. In this way, the Fe2+ ion achieves its most stable (lowest energy) electronic structure. The complex is diamagnetic, with no unpaired electrons. 

A major weakness of valence bond theory has been its inability to predict the magnetic properties of molecules. We mentioned this problem in Chapter 7 with regard to the 02 molecule, which is paramagnetic, even though it has an even number (12) of valence electrons. The octet rule, or valence bond theory, would predict that all the electrons in 02 should be paired, which would make it diamagnetic. 

The diamagnetism of all these rhodium(II) compounds indicates spin pairing via a metal-metal bond. 

The observed diamagnetism of the ions [Mo(CN)s]4- and [W(CN)8]4 shows that the central atom forms eight electron-pair bonds, involving the eigenfunctions dispi (fourth column of Table III). 

The metal carbonyls Ni(CO)4, Fe(CO)s, and Cr(CO)6 are observed to be diamagnetic. This follows from the theoretical discussion if it is assumed that an electron-pair bond is formed with each carbonyl for the nine eigenfunctions available (3d64s4p3) are completely filled by the n bonds and 2(9 — n) additional electrons attached to the metal atom (n = 4, 5, 6). The theory also explains the observed composition of these unusual sub-... 

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