16 valence electrons, fragments

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]. 

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. 

Obviously, the 12-valence-electron fragment [Hf(-rf-cot)], which forms by loss of butadiene, is an ideal template for the accommodation of more than two phosphaalkyne molecules. As illustrated later, the 14-electron fragment [HfCp2] only allows cyclodimerization of phosphaalkynes, and similar observations have been made with electron-rich transition metals such as Ni, Co, Rh, and Fe. 

A1 (CH2 0113)3 1 indicates that aluminum bonds to three CH2 CH3 fragments. There are 42 valence electrons, all of which are used to complete the bonding framework. Each of the six carbon atoms in triethylaluminum has an octet of electrons and a steric number of 4. Thus, each ethyl group of A1 (CH2 0113)3 described exactly... 

Within the computational scheme described in the course of this work, the available information about the atomic substructure (core+valence) can be taken into account explicitly. In the simplest possible calculation, a fragment of atomic cores is used, and a MaxEnt distribution for valence electrons is computed by modulation of a uniform prior prejudice. As we have shown in the noise-free calculations on l-alanine described in Section 3.1.1, the method will yield a better representation of bonding and non-bonding valence charge concentration regions, but bias will still be present because of Fourier truncation ripples and aliasing errors ... 

Atoms of S and Se can sufficiently structurally influence fragments of CH3 that are frequently located on the ends of hydrocarbon chains or in the form of free radicals. The data given confirm high reactivity of sulfur and selenium atoms as retardants of chain reactions of free radicals as elements drawing back impaired valence electrons of free radicals, but at the same time preserving the basic structure of hydrocarbon chain. 

Some general comments on the solid-state chemistry ( From a molecular view on solids to molecules in solids ) have been reported by Simon (1995) emphasis was especially placed on the structural chemistry of metal-rich compounds formed by the metals in groups 1 to 6 and it was underlined that it is largely based on discrete and condensed clusters. In the chemistry of metals in low oxidation states, the residual valence electrons can be used for metal—metal bonding. Metal-rich compounds lie between normal valence compounds and the elemental metals themselves, with respect to their compositions, and often also with respect to their structures fragments of usual metal structures (close-packed, b.c.c., etc.) are often component units in the structures of metal-rich compounds. 

Consequences of Increasing the Number of Valence Electrons on the Main-Group Fragment... 

Unlike the Sn6[M(CO)s]6 clusters, the Sn6Nb2(toluene)2 ion contains an open cyclohexane-like Sns ring that can be viewed as an Sns fragment if all of the Nb valence electrons are partitioned to Sn according to the Zintl formalism [43]. The six Sn atoms are equivalent by symmetry giving rise to a single resonance at 8... 

SRPA has been already applied for atomic nuclei and clusters, both spherical and deformed. To study dynamics of valence electrons in atomic clusters, the Konh-Sham functional [14,15]was exploited [7,8,16,17], in some cases together with pseudopotential and pseudo-Hamiltonian schemes [16]. Excellent agreement with the experimental data [18] for the dipole plasmon was obtained. Quite recently SRPA was used to demonstrate a non-trivial interplay between Landau fragmentation, deformation splitting and shape isomers in forming a profile of the dipole plasmon in deformed clusters [17]. 

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. 

Triatomic fragments involving boron and carbon, such as those found in many borocarbide structures,16 show typical valence electron counts ranging between 12 and 16 electrons. These counts are unusual for isolated triatomic molecules, which usually have 16 or more electrons, unless one of the terminal atoms is FI (12 electrons).17 Within the boron-carbon family, four of six possible arrangements for the trimeric fragments have been observed (1) C—C-C, (2) C-B-C, (3) B-B—C, and (4) B-C-B. Coincident-ally, only those with D,h point symmetry occur in metal borocarbides. They include (1) CCC units in Sc3C4 18 (2) CBC units in Si BCj 19,20... 

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). 

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