Tetrahedral species

However, with the application in the 19, iOs of crystal held theory to transition-metal ehemistry it was realized that CFSEs were unfavourable to the lormation of tetrahedral d complexes, and previous assignments were re-examined. A typical ca.se was Ni(acac)i. which had often been cited as an example of a tetrahedral nickel complex, but which was shown - in I9. I6 to be trimeric and octahedral. The over-zealous were then inclined to regard tetrahedral d" as non-existent until Hrst L.. M. Venanz.i and then N., S. Gill and R. S. Nyholm" demonstrated the existence of discrete tetrahedral species which in some cases were also rather easily prepared. 

Consider now spin-allowed transitions. The parity and angular momentum selection rules forbid pure d d transitions. Once again the rule is absolute. It is our description of the wavefunctions that is at fault. Suppose we enquire about a d-d transition in a tetrahedral complex. It might be supposed that the parity rule is inoperative here, since the tetrahedron has no centre of inversion to which the d orbitals and the light operator can be symmetry classified. But, this is not at all true for two reasons, one being empirical (which is more of an observation than a reason) and one theoretical. The empirical reason is that if the parity rule were irrelevant, the intensities of d-d bands in tetrahedral molecules could be fully allowed and as strong as those we observe in dyes, for example. In fact, the d-d bands in tetrahedral species are perhaps two or three orders of magnitude weaker than many fully allowed transitions. 

LFSE s for tetrahedral species are computed in a similar manner. They are compared with the results for octahedral systems in Fig. 8-7. No illustration of LFSE s for low-spin tetrahedral ions is included here because, as noted in Chapter 5, the much smaller values of Aet relative to oct ensures that pairing energies P always outweigh the ligand-field terms in practice. 

Taking all of these results together, some general patterns emerge. Foremost, the tetrahedral adducts 1 are found to be energy minima when the substituents X and Y are both first-row elements. However, when X and Y are both second-row elements, the tetrahedral species is a transition state and the only minima are ion-dipole complexes, 1. Clearly, two key factors in the formation of the tetrahedral adduct 1 are the difference in gas-phase basicities for the two anions (X" and 1) and the difference... 

Properties of nickel poly(pyrazol-l-yl)borate complexes such as solubility, coordination geometry, etc., can be controlled by appropriate substituent groups on the pyrazol rings, in particular in the 3- and 5-positions. Typical complexes are those of octahedral C symmetry (192)°02-604 and tetrahedral species (193). In the former case, two different tris(pyrazolyl)borate ligands may be involved to give heteroleptic compounds.602,603 Substituents in the 5-position mainly provide protection of the BH group. Only few representative examples are discussed here. 

Zn and N K-edge XANES have been used to distinguish the coordination geometries in (1,2-ethanediylidene)-bis(5 -methylhydrazonecarbodithionate) zinc complexes (109). The technique distinguished between tetrahedral species, square pyramidal dimers, and square pyramidal monomers, formed when pyridine was present. These studies were in conjunction with spectroscopic characterization and X-ray single-crystal data where possible.53 The results demonstrated the value of this technique when single crystals could not be obtained. 

The homologation of selenoesters 379 with diazomethane in the presence of Cu or Cul to give a-selenoketones is thought not to involve a carbenoid pathway and an Se-ylide intermediate but rather a tetrahedral species resulting from nucleophilic attack of CH2N2 at the carbonyl carbon atom. The role of the catalyst is seen in facilitating nucleophilic attack at C=0 by complexation at the selenium atom. 

The above results mainly apply to the Longuet-Higgins E x e problem, but this historical survey would be incomplete without reference to early work on the much more challenging problems posed by threefold or higher electronic degeneracies in molecules with tetrahedral or octahedral symmetry [3]. For example, tetrahedral species, with electronic symmetry T or T2, have at least five Jahn-Teller active vibrations belonging to the representations E and T with individual coordinates (Qa,Qb) and (Qx. Qx. Q ) say. The linear terms in the nine Hamiltonian matrix elements were shown in 1957 [3] to be... 

Predicted Uranium Tetra-auride and Other M(Au)4 Tetrahedral Species (M = Ti, Zr, Hf, Th). 

Kotaka M, Kakihana H (1977) Thermodynamic isotope effect of trigonal planar and tetrahedral species. Bull Research Lab Nuc Reactors 2 13-29... 

Rudolph and his coworkers also reported observation by Sn and Sn NMR of what was Interpreted to be a novel, fluctlonal tetrahedral Sni 2- species when they used Na-Sn alloys poorer In tin, comparable to those used in the preparation of the highly Insoluble (cp-Na" )2Sn5 2-. diamagnetic "tetrahedral species... 

Organoboranes have been shown to undergo fragmentation if a good leaving group is present on the 6 carbon.50 The electron donor is the tetrahedral species formed by addition... 

This conclusion was verified by a C NMR experiment carried out in 2h2 8o which gave a 0.05 ppm upfield shift in the resonance for the C-3 carbon relative to the carbon resonance in 2H2 0. The upfield shift in the carbon resonance establishes that the oxygen nucleophile that adds to the C-3 carbonyl group when 6 binds to pepsin must come from water. These labeling results are not consistent with the addition of the Asp-32 carboxyl group to the carbonyl group to form a covalent tetrahedral species as would occur during nucleophilic catalysis. 

The line-shape of this tetrahedral species does not change after evacuation of the sample or exposure to moisture, whereas upon evacuation the submonolayer tetrahedral V species detected on Si02 (7) or other oxides easily coordinate water molecules after exposure of the samples to ambient conditions and thus reform the more stable octahedral coordination. This indicates that vanadium is stabilized in nearly tetrahedral coordination by a direct specific interaction with the silicalite framework and the interaction is stronger than that observed for vanadium supported on silica. 

Nature of Vanadium Species. The data on the characterization of V- containing silicalite indicate the presence of various types of vanadium species (i) a polynuclear V-oxide containing V in various valence states (V , V and V ), (ii) octahedral sites, preferentially interacting with OH groups localized inside the pore structure of the zeolite crystals, (iii) nearly symmetrical tetrahedral species, attributed to a V species directly interacting with the zeolite framework, and (iv) after reduction, V species in a nearly tetrahedral environment. 

In the last few years the phenomenon has been extensively studied by Sacconi and his colleagues, who have investigated the physical properties of a series of bis(N-alkylsalicylaldimine)nickel compounds the alkyl chains have always been unbranched. Measurements of atom polarizations (221a, 225) and of dipole moments (225) indicate that there are no tetrahedral species in these solutions the dipole moments of both diamagnetic and of paramagnetic species are almost certainly zero, and so the tram-planar configuration (Du, symmetry) is common to both forms. The retention of D2h symmetry in the paramagnetic form does not exclude the possibility of solvent coordination 011 the z-axis. 

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