Electron precise systems

In the case of the octaosmium carbonyl, it is not clear whether the heptaosmium carbonyl anion has 20 or 21 carbonyl groups in the ion. If the latter is the case, then the ion corresponds to the "electron -precise system according to or the Wade theory, and arises from an "electron-deficient neutral carbonyl species. These reactions are of considerable preparative utility for the Os6/Os5 system, and, as has been amplified in the discussion of the pentaosmium carbonyl series, form a natural... 

Also shown in Figure 1.11(c), for purposes of comparison, are three neutral classically bonded hydrocarbons, propane, cyclopropane, and cyclopropene. For these systems, and for electron-precise systems in general, the number of electrons available for bonding (n) is equal to the number of AOs available (and so precisely the right number to fill the nil bonding MOs). 

In electron-precise systems, processes which do not change the nature of the bonding along the reaction path can often be computed with reasonable accuracy even at low levels of theory. For example, the rotational barrier of ethane, which... 

Electronic-Grade MMCs. Metal-matrix composites can be tailored to have optimal thermal and physical properties to meet requirements of electronic packaging systems, eg, cotes, substrates, carriers, and housings. A controUed thermal expansion space tmss, ie, one having a high precision dimensional tolerance in space environment, was developed from a carbon fiber (pitch-based)/Al composite. Continuous boron fiber-reinforced aluminum composites made by diffusion bonding have been used as heat sinks in chip carrier multilayer boards. 

A modem mass spectrometer is constmcted from elements which approach the state-of-the-art in solid-state electronics, vacuum systems, magnet design, precision machining, and computerized data acquisition and processing. [1] This is and has ever been a fully valid statement about mass spectrometers. 

Electron diffractometry system with the combination of the precession technique can be very perspective experimental instrumentation for precise structural investigations. The technique can now be adapted in a commercial TEM (previously applied uniquely to electron diffraction cameras) taking advantage of the small beam size and can measure reflections in the ED pattern with same required precision for structure analysis. 

An even broader range of ring sizes is found for the cyclo-phosphazenes (R2PN)x (x=3-1T) but these inorganic heterocycles are ir-electron precise (l.e. the number of ir-electrons is equal to the number of atomic centers in the ring system). Although both bicyclic and condensed structures have been identified, simple anions or cations of cyclophosphazenes are unknown ( ). ... 

For molecules with more than one electron, precise solutions become even more difficult and time consuming, and additional approximations are sought. The simplest molecule is that of hydrogen, where there are two nuclei A and B, and two electrons 1 and 2. The potential energy of the system is the sum of six electrostatic terms the four attractive terms between A-1, A-2, B-1, and B-2, and the two repulsive terms between A-B and 1-2. We seek solutions to the Schrodinger equation of this hydrogen molecule, and the solution is assumed to be a linear combination of the products of the atomic orbitals, of nucleus A associated with electron 1 multiplied by nucleus B associated with electron 2, plus nucleus A associated with electron 2 multiplied by... 

Electron counting in these electron-deficient systems is slightly different than in electron-precise hydrides. As noted by Green,8 canonical forms can be written in which the hydrogen is treated normally vis-a-vis one metal and the M—H bond is treated as a two-electron ligand to the second... 

Few-electron atomic systems cannot be precisely calculated at low Z, except some special cases (like e. g. isotopic shift). 

As a mutually bonded four-atom array is the smallest entity we consider to be a cluster, the EM3 system constitutes an example of the simplest cluster. As it is electron precise (the number of skeletal electron pairs is... 

Trinuclear dusters are nearly always triangular, often heteronuclear, and very numerous. The M3(CO)i2 clusters of Fe, Ru, and Os have been much studied and can be used to illustrate some typical cluster chemistry. The Os3(CO)i2 cluster with the structure (16-XIII), where CO groups are denoted simply by lines, are electronically precise, namely, they have exactly the right number of electrons to provide each metal atom with an 18-electron, closed shell configuration. In such systems there is a total of 48 electrons, and each M—M bond is of order 1. As... 

Electron-precise cluster systems can be seen to be a more complex version (three-connect units) of chains (one-connect units) and rings (two-connect units), leading up to an extended solid (four-connect units, e.g. the diamond structure). This is most easily appreciated for homonuclear examples, for example, (1) and (2). For the mixed systems like (3), two distinct descriptions are possible - as a 4-atom cluster or as a trinuclear metal complex with a /U.3-P cap. In many cases, the difference is of little consequence however, in others, proper choice of description can lead to analyses of significant conceptual value. 

PNMR spectroscopy and generates heterocycles that are hybrids of r-electron-rich (S-N) and tt-electron-precise ring systems (cyclophosphazenes, see Phosphorus-Nitrogen Compounds). 

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