Compounds with Bond Order Four

Because of orbital availability, bonding with order four is possible only between transition metals. Orbitals with angular quantum momentum number 2 or higher are necessary. 

An acceptable description of multiple metal-metal bond can be obtained indeed by considering the overlap between d-orbitals of two adjacent metal atoms mentioned above. As described in Fig. 1.29, d atomic orbital overlapping occurring between two metal atoms at bond distances lead to molecular orbitals of type (T, 7c, and 3. 

The first chemical species for which the existence of quadruple bond was determined is the anion [Re2Cl8] . In Fig. 1.33 the structure of the tert-butylamonium salt of this anion is reproduced. In this structure, as well as in data in Table 1.7 it can be appreciated that the Re-Re distance is considerable shorter than that in metallic rhenium (2.75 A). As observed in the structure in Fig. 1.33, chloride ligands do show an eclipsed configuration in spite of possible Cl-Cl interactions. 

Spectroscopic properties of species with quadruple metal-metal bonds agree very well with theoretical descriptions. Low energy absorptions in UV-spectra normally assigned to d-5 transitions as well as other transitions in the spectra accord well, at least qualitatively, with calculated MO schemes and confirm 

Although metal atoms in the octahalide derivatives appear to be coor-dinatively saturated, the tetracarboxilates M2(02CR)4 often show a certain 

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The P n.m.r, parameters have been tabulated for a wide range of P" amino-compounds and P compounds. The value of Sp for compounds with four P—N bonds correlates with the hybridization of the nitrogen atom, moving to higher field in the order p < sp < sp < sp. 

For C70 and higher fullerenes addition also occurs only at the [6,6] fusions. However, these sites are no longer all equivalent, and a variety of regioisomers can result. For C70, of the four possible products, as illustrated in Fig. 10, addition occurs exclusively at the polar fusion, A (,14). This is the sterically most accessible site, and energy calculations have shown there is the greatest release of steric strain on com-plexation (91). The equatorial [6,6] fusion, D, has lowest bond order, and correspondingly, no complexations to it have been observed. For C84, complexation with Vaska s compound occurs at the fusion with the highest bond order, as shown by Htickel calculations (28). 

The purpose of this review is to present a current description of the chemistry of compounds with quadruple metal—metal bonds and closely related species characterized through early 1978. Species containing metal-metal bonds of order four are considered to the exclusion of units containing single, double, and triple-bonds except where structural comparisons or chemical reactivity patterns dictate the presentation of data for the lower bond orders. 

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