16 valence electrons, fragments complexes

The coordination chemistry of 1,2,3-diazaphospholes has been scarcely developed. It features a few P- and N-coordinated complexes where the ring invariantly donates two electrons by virtue of the respective lone pair.30 Compounds 49a,b are the first transition-metal derivatives of 2H-1,2,3-diazaphospholes where a 17-valence electron fragment is linked to the nitrogen atom N(2) in place of an organosubstituent. 

The second octanuclear mixed chalcogen/carbonyl compound so far known is the complex anion [Fe8Te8(Te2)(CO)2o] . " " This novel cage system (Fig. 31) contains two cuboidal Fe4Te4(CO)ioTe2 fragments, which share the atoms of the ditelluride group. With a total of 144 valence electrons, the complex anion is electron-precise without any iron-iron bonds. 

In 45 the copper(I) ion is tri-coordinate and shows a planar surrounding set-up by the t -coordinated Ti(C=CSiMe3)2 unit and the datively-bonded (Ph3P)AuC=N building block counting to 16-valence electrons at the copper center, while in 46 the coordination number of silver(I) expands to four and an 18-valence electron fragment is formed [52]. Complex 46 displays a o -linear Ti-Ag-N=C-Au array as required by the pseudo-tetrahedral geometry around silver(I). For the different coordination behavior of copper(I) and silver(I) towards Lewis-bases see refs. [14] and [53]. 

These complexes are considered according to the mode of bonding of the hydrazido fragment. The known coordination types are shown in valence bond representations in Figure 15 together with the formal charge, geometry and numbers of valence electrons donated to the metal. 

The complementary approach, activation of unsaturated hydrocarbons toward electrophilic attack by complexation with electron-rich metal fragments, has seen limited investigation. Although there are certainly opportunities in this area which have not been exploited, the electrophilic reactions present a more complex problem relative to nucleophilic addition. For example, consider the nucleophilic versus electrophilic addition to a terminal carbon of a saturated 18-electron metal-diene complex. Nucleophilic addition generates a stable 18-electron saturated ir-allyl complex. In contrast, electrophilic addition at carbon results in removal of two valence electrons from the metal and formation of an unstable ir-allyl unsaturated 16-electron complex (Scheme 1). 

When the number of metal atoms in a cluster increases, the geometries of the clusters become more complex, and some are often structurally better described in terms of capped or decapped polyhedra and condensed polyhedra. For example, the first and second clusters listed in Table 19.4.3 are a capped octahedron and a bicapped octahedron, respectively. Consequently, capping or decapping with a transition-metal fragment to a deltapolyhedral cluster leads to an increase or decrease in the cluster valence electron count of 12. When a transition-metal atom caps a triangular face of the cluster, it forms three M-M bonds with the vertex atoms, so according to the 18-electron rule, the cluster needs an additional 18 - 6 = 12 electrons. The parent octahedron of [Os6(CO)is]2- has g = 86, the monocapped octahedron Osy(CO)2i has g = 98, and the bicapped octahedron [Oss(CO)22]2- hasg = 110. 

Considered as a c/oso-pentagonal bipyramid, the compound should possess eight sep however, with Mo(CO)3 + 5 Pb + 4- = (4 x 0 + 5 x 2 + 4)/2 = 7 sep assuming each Pb has an external lone pair and the metal fragments have filled t2g orbitals. Considered as a triple-decker complex it has 6 + 6 + 0 + 6 + 6 + 4 = 28 valence electrons and one would expect a Mo-Mo bonding interaction based on the arsenic model mentioned. However, the Mo-Mo distance is clearly non-bonding. In Chapter... 

In contrast, Schrock carbenes are electron deficient [10 to 16 valence electrons (VE)] early transition metal complexes with the metal atom in a high oxidation state and carbene substituents that are limited to alkyl groups and hydrogen [131]. Their bonding situation can be described in terms of the interaction of a triplet carbene with a triplet metal fragment resnlting in a covalent double bond [132], Tantalum complexes like [(np)3Ta=CHBu ] and [Cp2(Me)Ta=CH2] are representative of Schrock carbenes. 

Ruthenium has played a central role in the development of 30 valence electron triple-decker cations of the iron group. These compounds were first prepared by Rybinskaya eto/., through reaction of a 12 valence electron [CpRu]+ fragment with a metallocene. The well-known photofragmentation of [CpFe(arene)]+ was used to generate [FeCp]+, which was then complexed to ruthenocene in situ. The stmcture of the triple-decker shows three exactly parallel rings, two of them (one outer and the inner) echpsed and... 

---