Electronic complex atoms

For trinuclear cluster complexes, open (chain) or closed (cycHc) stmctures are possible. Which cluster depends for the most part on the number of valence electrons, 50 in the former and 48 in the latter. The 48-valence electron complex Os2(CO)22 is observed in the cycHc stmcture (7). The molecule possesses a triangular arrangement of osmium atoms with four terminal CO ligands coordinated in a i j -octahedral array about each osmium atom. The molecule Ru (00) 2 is also cycHc and is isomorphous with the osmium analogue. 

For many species the effective atomic number (FAN) or 18- electron rule is helpful. Low spin transition-metal complexes having the FAN of the next noble gas (Table 5), which have 18 valence electrons, are usually inert, and normally react by dissociation. Fach normal donor is considered to contribute two electrons the remainder are metal valence electrons. Sixteen-electron complexes are often inert, if these are low spin and square-planar, but can undergo associative substitution and oxidative-addition reactions. 

Charge-Transfer Forces. An electron-rich atom, or orbital, can form a bond with an electron-deficient atom. Typical examples are lone pairs of electrons, eg, in nitrogen atoms regularly found in dyes and protein and polyamide fibers, or TT-orbitals as found in the complex planar dye molecules, forming a bond with an electron-deficient hydrogen or similar atom, eg, —0 . These forces play a significant role in dye attraction. 

Boron is unique among the elements in the structural complexity of its allotropic modifications this reflects the variety of ways in which boron seeks to solve the problem of having fewer electrons than atomic orbitals available for bonding. Elements in this situation usually adopt metallic bonding, but the small size and high ionization energies of B (p. 222) result in covalent rather than metallic bonding. The structural unit which dominates the various allotropes of B is the B 2 icosahedron (Fig. 6.1), and this also occurs in several metal boride structures and in certain boron hydride derivatives. Because of the fivefold rotation symmetry at the individual B atoms, the B)2 icosahedra pack rather inefficiently and there... 

Each of the sandwich compounds forms two isomers, described as clockwise and counterclockwise, respectively. Clockwise means that the atomic sequence in both rings is the same, counterclockwise that the atomic sequence is opposite. The syntheses occur best in THE at -78°C. After warming, the solvent is removed. Purification can be carried out by crystallization from petroleum, ether or better by sublimation at 60-70°C and 10 torr. The yields vary between 25 and 85%. The 17-and 18-electron complexes with V and Fe atoms show the metal atoms to be fixed above and below the ring centers. In contrast, the 19- and 20-electron complexes of Co and Ni possess slipped rings. 

There are two general conclusions of importance. First, the distance r(Z- X), where Z is the electron donor atom/centre in the complex B- XY, is smaller than the sum of the van der Waals radii ax and ax of these atoms. This result has been shown [179] to be consistent with the conclusion that the van der Waals radius of the atom X in the dihalogen molecule X is shorter along the XY internuclear axis than it is perpendicular to it, i.e. there is a polar flattening of the atom X in the molecule XY of the type suggested by Stone et al. [180]. This result has been shown to hold for the cases XY = CI2 [174], BrCl [175], C1F [176] and IC1 [178], but not for F2, in which the F atom in the molecule appears (admittedly on the basis of only a few examples) to be more nearly spherical [177]. 

In addition to halogen bonded complexes or ionic salts, it is also possible for sulfur and selenium electron donors to form complexes in which the electron donor atom inserts into the X2 bond, giving a hypervalent donor atom with a T-shaped geometry. It has been recently reported [147] that for dibromine and selenium, this type of complex is favored over halogen bonded complexes. While no purely halogen bonded complex is reported for dibromine, there is one complex (IRABEI) in which one selenium atom of each of several selenanthrene molecules in the asymmetric unit does insert into a Br2 bond, but for one of the molecules, the other selenium atom forms a halogen bond with a Br2 molecule to form a simple adduct (A). 

We have already mentioned a very strong dyadic association in the formally d5 cobalt complexes such as [Cp Co(dddt)]+ which dimerizes in the solid state to a fully diamagnetic dicationic dyad (Fig. 6a). It represents the extreme situation where the two radicals form a true 2e bond, with the sulfur atom of one dithiolene ligand entering the coordination sphere of the other metal. It should be considered as the consequence of the electron deficiency of these cationic [CpCo(dt)]+ 15-electron complexes. 

The universal function x(x) obtained by numerical integration and valid for all neutral atoms decreases monotonically. The electron density is similar for all atoms, except for a different length scale, which is determined by the quantity b and proportional to Z. The density is poorly determined at both small and large values of r. However, since most electrons in complex atoms are at intermediate distances from the nucleus the Thomas-Fermi model is useful for calculating quantities that depend on the average electron density, such as the total energy. The Thomas-Fermi model therefore cannot account for the periodic properties of atoms, but provides a good estimate of initial fields used in more elaborate calculations like those to be discussed in the next section. 

The most obvious defect of the Thomas-Fermi model is the neglect of interaction between electrons, but even in the most advanced modern methods this interaction still presents the most difficult problem. The most useful practical procedure to calculate the electronic structure of complex atoms is by means of the Hartree-Fock procedure, which is not by solution of the atomic wave equation, but by iterative numerical procedures, based on the hydrogen model. In this method the exact Hamiltonian is replaced by... 

The same role that H plays in the theory of complex atoms may be expected for Hj as the prototype from which to generalize electron configurations of complex molecules. The molecular generalization must clearly... 

Fotiadis D, Mueller DJ, Tsiotis G, Hasler L, Tittmann P, Mini T, Jeno P, Gross H, Engel A. Surface analysis of the photosystem I complex by electron and atomic force microscopy. J Mol Biol 1998 283 83-94. 

Exceptions to the E.A.N. rule do occur, particularly with d8 metal ions, where many examples of square-planar, 16-electron complexes are known. In these complexes, the high-lying pz orbital is nonbonding and remains empty. This deviation from the rule is often said to be due to the large s-to-p promotion energies found for the free atoms. As the atomic number increases across a given transition-metal series, the... 

In contrast to the above-described systems, there are only few systems in which the bound cation can interact with the acceptor part of charge-transfer probes. The case of coumarins linked to crowns (Figure 10.24) is of special interest because the cation interacts directly with the electron-withdrawing group, i.e. the carbonyl group, in spite of the spacer between the fluorophore and the crown. An important consequence is the increase in stability constant of the complexes with respect to the same crown without external complexing atoms. 

The Hume-Rothery phases constitute an interesting and ubiquitous group of binary and complex intermetallic substances it was indeed Hume-Rothery who, already in the twenties, observed that one of the relevant parameters in rationalizing compositions and structures of a number of phases is the average number of valence electrons per atom (nJnM). An illustration of this fact may be found in Table 4.6, where a number of the Hume-Rothery structure types have been collected, together with a few more major structure types relevant to transition metal alloys. For each phase the corresponding VEC has been reported as njnai ratio, both calculated on the basis of the s and p electrons and of s, p and d electrons. 

Transition metal carbene complexes have broadly been classified into Fischer-type and Schrock-type carbene complexes. The former, typically low-valent, 18-electron complexes with strong 7t-acceptors at the metal, are electrophilic at the carbene carbon atom (C ). On the other hand, Schrock-type carbene complexes are usually high-valent complexes with fewer than 18 valence electrons, and without n-accepting ligands. Schrock-type carbene complexes generally behave as carbon nucleophiles (Figure 1.4). 

---