Isoelectronic fragments

The isolobal definition may be extended to isoelectronic fragments having the same coordination number. For example,... 

As discussed earlier (Section IV.I.Lh.ii), the insertion of sulfur into the zinc-sulfur bonds of [Zn(S2CNMe2)2] is proposed to be a key step in the rubber vulcanization process that utilizes this material. However, while the insertion of sulfur into the metal-sulfur bonds of related xanthate complexes is known, to date no such insertion has been noted for dithiocarbamates. However, the insertion of related isoelectronic fragments into the metal-sulfur bond(s) is known. 

Another important concept is the notion of stabilization by means of coordination, A classic example is the. stabilization of the fugitive species cyclobutadiene, C4H4I by coordination to (Fe(CO)3l (p. 936). As the C atom is isoclectronic with (BH], so (C4H4] is isoelectronic with the borane fragment (BaH which is similarly stabilized by coordination to (Fe(CO)3l or the isoelectronic (Cofr/ -CsHs)) (see Panel on p. 174), Indeed it is a general feature of metallaboranc chemistry that such clusters are often much more stable than are the parent boranes themselves. 

Neutral 2,3-dihydro-lH-l,3)2-diazaboroles are isoelectronic with the cyclopenta-dienyl anion and, therefore, prefer to react with neutral transition-metal fragments having 12 electrons ... 

The same strategy as for the synthesis of 40 has been employed for the preparation of the Li/Sn-mixed cluster 41. Thus, replacement of the imido groups in [Sn(NtBu)]4 with LiPHR (R = cyclohexyl) in the molar ratio of 4 6 yielded the metallacyclic cage complex 41, which has a rhombododecahedral Li4Sn4P6 core (Eq. 24) (69). The clusters 40 and 41 are isostructural, since the MeAl fragments in 40 have been replaced by the isoelectronic Sn(II) centers. 

They are formally isoelectronic with the (ArH)Cr(CO)3 series, and are derived from Co4(CO)12. The thermal decomposition of three representatives of the series has been studied by microcalorimetry84) and the results are shown in Table 16. Once again heats of sublimation have had to be estimated by comparison with the chromium analogues. The enthalpy disruption can be divided by taking T = 134 kJ mol-1 (Table 1) so that the b.e.c of the [Co4(CO)9] fragment in Co4(CO)i2 is 1722 kJ mol-1. The (ArHCo) bond enthalpy contribution is then obtained in the usual way the results are shown in Table 16. It is clear that as in the chromium series, the b.e.c (ArH-Co) increases along the series benzene < mesitylene < hexamethylbenzene. 

In 1971, a note 164) was published favoring the hypothesis that the carboranes, boranes, their isoelectronic anions, Lewis base adducts, and heteroatom-substituted analogs should be viewed as constructed about the vertices of either the most spherical series of triangular-faceted polyhedra (deltahedra) found to be characteristic of the dicarba-cZoao-carboranes (Fig. 1) or, with one lone exception, fragments of the series of deltahedra produced by the successive removal of the highest coordinated vertices that sequentially define the nido and arachno classes. This position was in conflict with the then prevalent shibboleth that all nido and arachno compounds [except B5H9 (I-N5)] had or would prove to have icosahedral fragment structures. 

A number of studies published since 1971 confirm several of the initial predictions. Two groups at Indiana University [Siedle et al. 128) and Streib et al. (62)] have revealed that the nine-vertex nido structures B9Hi2 (II-N9) and the isoelectronic C, C-dimethyl derivative of C2B7H11 (IX-N9) do have nonicosahedrcd fragment strnctures (see Figs. 1 and 9 ... 

There is an exception in addition to the expected normal arachno nine-vertex fragment characteristic of w-BqHis (XI-A19) (75, ISO), there is a fragment generated by the removal of a low coordination vertex which is reflected in the structures of both i-ByHis (I-A19) 21) and its isoelectronic analog i-C2B7Hi3 (II-A19) 1S6). 

The nido ten-vertex and arachno ten-vertex deltahedral fragments are coincidently the same. Such ten-vertex nido and arachno species are not isoelectronic, although they are frequently misjudged as being isoelectronic, and a number of investigators have assigned incorrect structures based on comparison of B NMR spectra of compounds that were mistakenly thought to be isoelectronic. 

Fig. 28. Substitution of isolobal and isoelectronic CH and M(CO)3 (M = Co or Ir) fragments in tetrahedral clusters. It is interesting that the final member of the series C4H4 (tetrahedrane) has not been isolated.

This division of odd AH anions into an lOAH anion fragment and cross-conjugated even AH substituents is a convenient simplification that will be employed in the following sections to discuss isoelectronic heterocycles. However, it must not be forgotten when considering heteroderivatives that... 

---