Transition, bond type

Attempts to classify carbides according to structure or bond type meet the same difficulties as were encountered with hydrides (p. 64) and borides (p. 145) and for the same reasons. The general trends in properties of the three groups of compounds are, however, broadly similar, being most polar (ionic) for the electropositive metals, most covalent (molecular) for the electronegative non-metals and somewhat complex (interstitial) for the elements in the centre of the d block. There are also several elements with poorly characterized, unstable, or non-existent carbides, namely the later transition elements (Groups 11 and 12), the platinum metals, and the post transition-metal elements in Group 13. 

It is then shown that (excepting the rare-earth ions) the magnetic moment of a non-linear molecule or complex ion is determined by the number of unpaired electrons, being equal to ms = 2 /S(S + 1), in which 5 is half that number. This makes it possible to determine from magnetic data which eigenfunctions are involved in bond formation, and so to decide between electron-pair bonds and ionic or ion-dipole bonds for various complexes. It is found that the transition-group elements almost without exception form electron-pair bonds with CN, ionic bonds with F, and ion-dipole bonds with H2O with other groups the bond type varies. 

In such cases as these it is evident that a continuous transition from one extreme structure to another could occur. If, however, the structures have different multiplicities, they cannot be combined with one another (so long as spin-orbit interactions are negligible), so that the transition from one extreme bond type to the other would be effectively discontinuous. 

The Transition from One Extreme Bond Type to Another... 

A question which has been keenly argued for a number of years is the following if it were possible continuously to vary one or more of the parameters determining the nature of a system such as a molecule or a crystal, say the effective nuclear charges, then would the transition from one extreme bond type to another take place continuously, or would it show discontinuities For example, are there possible all intermediate bond types between the pure ionic bond and the pure electron-pair bond With the development of our knowledge of the nature of the chemical bond it has become evident that this question and others like it cannot be answered categorically. It is necessary to define the terms used and to indicate the point of view adopted and then it may turn out, as with this question, that no statement of universal application can be made. 

After a discussion of the properties of substances containing ionic bonds and electron-pair bonds, it is shown that the transition from one extreme bond type to another could take place continuously in some cases (when... 

This means that the ionization and rearrangement need not be concerted and that symmetrical protonated ethylene can not be a major intermediate in the reaction. A similar experiment with isobutylamine and nitrous acid in heavy water gave products that contained no carbon-deuterium bonds. Since it is known that the -complex formed from isobutylene and acid is in rapid equilibrium with protons from the solvent, none of this can be formed in the nitrous acid induced deamination. This in turn makes it probable that the transition state for the hydrogen migration is of the sigma rather than the -bonded type.261... 

In practice, the NBO program labels an electron pair as a lone pair (LP) on center B whenever cb 2 > 0.95, i.e., when more than 95% of the electron density is concentrated on B, with only a weak (<5%) delocalization tail on A. Although this numerical threshold produces an apparent discontinuity in program output for the best single NBO Lewis structure, the multi-resonance NRT description depicts smooth variations of bond order from uF(lon) = 1 (pure ionic one-center) to bu 10n) = 0 (covalent two-center). This properly reflects the fact that the ionic-covalent transition is physically a smooth, continuous variation of electron-density distribution, rather than abrupt hopping from one distinct bond type to another. 

Linus Pauling, "The Nature of the Chemical Bond. III. The Transition from One Extreme Bond Type to Another," JACS 54 (1932) 981003 Linus Pauling, "Interatomic Distances in Covalent Molecules and Resonance between Two or More Lewis Electronic Structures," Proc.NAS 18 (1932) 293297 Linus Pauling, "The Calculation of Matrix Element for the Lewis Electronic Structure of Molecules,"... 

The Nature of the Chemical Bond. III. The Transition from One Extreme Bond Type to Another." JACS 54 (1932) 981003. 

In 1995 a breakthrough occurred in this field in February we were able to show the synthesis and the spectroscopic properties of the first complexes of type B [7], and in August the first isolated and structurally characterized complexes containing terminal metal-phosphorus triple bonds (type A) were independently obtained and published in back-to-back articles by the groups of Cummins [8] and Schrock [9]. Since then, a rapid development has occurred in the synthesis and particularly in the study of the reactivity pattern of complexes with phosphorus-transition metal triple bonds. This review chapter will highlight the development in this field by giving an overview from 1995 until the current stage of research. 

THE TRANSITION FROM ONE EXTREME BOND TYPE TO ANOTHER... 

Discontinuous Change in Bond Type.5—In molecules and complex ions of certain types continuous transition from one extreme bond type to another is not possible. In order for continuous transition to be possible between two extreme bond types the conditions for resonance between the corresponding structures must be satisfied. The most important of these conditions is that the two structures must involve the same numbers of unpaired electrons. If the two structures under consideration involve different numbers of unpaired electrons, then the transition between the two must be discontinuous, the discontinuity being associated with the pairing or unpairing of electrons ... 

These complexes and others of similar character are discussed further in Chapter 5, in which a magnetic criterion for bond type applicable to complexes of the transition elements is described. 

The hybrid bond orbitals discussed in the preceding chapter have been described as having only a small amount of d and / character. The bonds in many molecules and complex ions, especially those involving atoms of the transition elements, can be discussed in a simple way in terms of hybrid orbitals with a large amount of d character (and, in a few cases, / character). These bonds and a magnetic criterion for bond type are discussed in the following sections. 

Hybrid S values for any desired ty >e of hybrid can be obtained very easily from the tables as simple linear combinations of nonhybrid 5 values. It is shown how 5 values corresponding to orthogonalizerl Slater AO s and approximate S values for SCF (self-consistent-field) AO s can also be obtained as linear combinations of the Slater-AO S values. 5 values for carbon-carbon 2pa- and 2/r-bonds using SCF carbon AO s have been computed (see Table in Section Vb) they correspond to stronger overlap than for Slater AO s. Non-locaHaed MO group-orbital S values are also discussed, and are illustrated by an application to H20. The use of the tables to obtain dipole moments for electronic transitions in certain cases is also mentioned. The use of the tables to obtain S values for various specific atom-pairs and bond-types, and resulting conclusions, will be discussed in another paper. 

Carbon disulfide forms complexes in which the metal has a low oxidation state with almost every transition metal. The complexes have been reviewed extensively.1 7,8 Three bonding modes are found end-on via S, if bonded and bridging between two metal atoms. The evidence for these three bonding types is largely spectroscopic and therefore limited. CS2 shows a variety of insertion and disproportionation reactions. 

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