Two-coordinate molecules

Roesky used 7 to prepare heteroleptic calcium (35), strontium (36), and barium (37) complexes from the corresponding metal diiodides.32 Complex 35 was found to be monomeric, exhibit a calcium-methanide contact, and to be isostructural to the analogous ytterbium(II) complex 93 (vide infra). In contrast, complexes 36 and 37 were found to be dimeric, the former is isostructural to the samarium and europium analogues 91 and 92 (vide infra), and constructed around four-membered M2l2 rings but the larger size of barium compared to strontium is reflected by the inclusion of one coordinated THF molecule per strontium in 36 but two coordinated molecules of THF per barium in 37. Both 36 and 37 exhibit metal-methanide contacts which for 37 contrasts with the situation in 34. Complex 38 was prepared from two equivalents of 7 and strontium diiodide and, in contrast to 34, was found to exhibit metal-methanide contacts.33... 

For the interaction between a nonlinear molecule and an atom, one can place the coordinate system at the centre of mass of the molecule so that the PES is a fiinction of tlie three spherical polar coordinates needed to specify the location of the atom. If the molecule is linear, V does not depend on <() and the PES is a fiinction of only two variables. In the general case of two nonlinear molecules, the interaction energy depends on the distance between the centres of mass, and five of the six Euler angles needed to specify the relative orientation of the molecular axes with respect to the global or space-fixed coordinate axes. 

There is much discussion on the nature of the aluminum species present in slightly acidic and basic solutions. There is general agreement that in solutions below pH 4, the mononuclear Al " exists coordinated by six water molecules, ie, [ ( 20) ". The strong positive charge of the Al " ion polarizes each water molecule and as the pH is increased, a proton is eventually released, forming the monomeric complex ion [A1(0H)(H20) ]. At about pH 5, this complex ion and the hexahydrated Al " are in equal abundance. The pentahydrate complex ion may dimerize by losing two water molecules... 

Monosodium L-glutamate [142-47-2] C HgNO Na H20 (mol wt 187.13) crystallizes from aqueous solution at room temperature as rhombic prisms. Its stmcture, as deterrnined by x-ray crystallography (6), indicates that the sodium ions are coordinated octahedraHy by four (3a and ly) carboxyl oxygen atoms and two water molecules as follows ... 

A hydroxyl group is situated ortho to a carboxyl group which as a bidentate ligand is terminally metallized on the fiber when aftertreated with dichromate. An example is Alizarine Yellow GG [584-42-9] (50) (Cl Mordant Yellow 1 Cl 14025). Cr(III) has a coordination number of six, and therefore normally two dye molecules of the sahcyhc type are chelated to the metal ion. 

Another group of macrocyclic ligands that have been extensively studied are the cycHc polyethers, such as dibenzo-[18]-crown-6 (5), in which the donor atoms are ether oxygen functions separated by two or three carbon atoms. The name crown ethers has been proposed (2) for this class of compounds because of the resemblance of their molecular models to a crown. Sandwich stmctures are also known in which the metal atom is coordinated with the oxygen atoms of two crown molecules. 

Figure 13.5 A Mg + atom links GTP to the Ras protein. Mg Is coordinated to one oxygen atom each from the p and y phosphates of GTP as well as to the side chains of Set 17 and Thr 35 of Ras. Two water molecules complete the octahedral coordination of Mg.

Crystal structure determination has also been done with -butyllithium. A 4 1 n-BuLi TMEDA complex is a tetramer accommodating two TMEDA molecules, which, rather than chelating a lithium, link the tetrameric units. The 2 2 -BuLi TMEDA complex has a structure similar to that of [PhLi]2 [TMEDA]2. Both 1 1 -BuLi THF and 1 1 -BuLi DME complexes are tetrameric with ether molecules coordinated at each lithium (Fig. 7.2). These and many other organolithium structures have been compared in a review of this topic. ... 

In the monomeric structure of 10.21 the solvation of both Li" ions by two THF molecules prevents further aggregation. The four S-N bond lengths are equal at ca. 1.60 A indicating that the negative charge is delocalized over the S(N Bu)4 unit. In the dimer 10.22 one [McS(N Bu)3] anion is coordinated to both Li" ions, one of which is bis-solvated by THF, while the other is chelated only to the unsolvated Li" ion. 

The very long 8-1 bonds in the linear 8-1-8 bridge (267.5 pm) are notable and have been interpreted in terms of an 8-1 bond order of f. Even weaker 8- T interactions occur in the cation [8214] + which could, indeed, alternatively be regarded as an 82 + cation coordinated side-on by two I2 molecules (Fig. 15.23). This... 

Once the electronic Schrodinger equation has been solved for a large number of nuclear geometries (and possibly also for several electronic states), the PES is known. This can then be used for solving the nuclear part of the Schrodinger equation. If there are N nuclei, there are 3N coordinates that define the geometry. Of these coordinates, three describe the overall translation of the molecule, and three describe the overall rotation of the molecule with respect to three axes. Eor a linear molecule, only two coordinates are necessary for describing the rotation. This leaves 3N-6(5) coordinates to describe the internal movement of the nuclei, the vibrations, often chosen to be... 

For bimolecular reactions (i.e. where the reactant is two separate molecules) and contribute a constant —4 RT. The translational and rotational enttopy changes are substantially negative, —30 to —50 e.u., due to the fact that there are six translational and six rotational modes in the reactants but only three of each at the TS. The six remaining degrees of freedom are transformed into the reaction coordinate and five new vibrations at the TS. These additional vibrations usually make a few kcal/mol... 

Reaction of diphenyl-2-thienylphosphine with Ru3(CO)l2 gives the tiVp) ti Ti C) coordinated species, 144, along with cluster 145 where two ligand molecules participate in coordination, one via the phosphorus atom and the C=C bond of the heteroring, and the other via the phosphorus atom only. P-Coordination in the products of such an interaction is known [95JOM(488)85]. Complex 144, the main product, interacts with carbon monoxide to yield the P-coordinated cluster, 146,... 

Finally, some orgatiocopper compounds undergo diarge disproporLionation under die influence of ligands dial bind strongly to copper. Treatment of mesityl-copper witli 1,2-bis-idipbenylpbospbinoJediane iDPPE), for example, results in tlie fotniation of bisimesityljcopper anions and a copper cation to wbidi four pbos-pborus atoms of two DPPE molecules ate coordinated [75]. 

The solvent used plays an important role, since it can stabilize the organomag-nesium species through complexation. Nucleophilic solvents such as ethers—e.g. diethyl ether or tetrahydrofuran—are especially useful. The magnesium center gets coordinated by two ether molecules as ligands. 

When, however, the ligand molecule or ion has two atoms, each of which has a lone pair of electrons, then the molecule has two donor atoms and it may be possible to form two coordinate bonds with the same metal ion such a ligand is said to be bidentate and may be exemplified by consideration of the tris(ethylenediamine)cobalt(III) complex, [Co(en)3]3+. In this six-coordinate octahedral complex of cobalt(III), each of the bidentate ethylenediamine molecules is bound to the metal ion through the lone pair electrons of the two nitrogen atoms. This results in the formation of three five-membered rings, each including the metal ion the process of ring formation is called chelation. 

Multidentate ligands contain more than two coordinating atoms per molecule, e.g. 1,2-diaminoethanetetra-acetic acid (ethylenediaminetetra-acetic acid, EDTA),f which has two donor nitrogen atoms and four donor oxygen atoms in the molecule, can be hexadentate. 

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