Tetrahedral metal centers, distorted

Fig. 2. Structures for the solid (a) fee Cco, (b) fee MCco, (c) fee M2C60 (d) fee MsCeo, (e) hypothetical bee Ceo, (0 bet M4C60, and two structures for MeCeo (g) bee MeCeo for (M= K, Rb, Cs), and (h) fee MeCeo which is appropriate for M = Na, using the notation of Ref [42]. The notation fee, bee, and bet refer, respectively, to face centered cubic, body centered cubic, and body centered tetragonal structures. The large spheres denote Ceo molecules and the small spheres denote alkali metal ions. For fee M3C60, which has four Ceo molecules per cubic unit cell, the M atoms can either be on octahedral or tetrahedral symmetry sites. Undoped solid Ceo also exhibits the fee crystal structure, but in this case all tetrahedral and octahedral sites are unoccupied. For (g) bcc MeCeo all the M atoms are on distorted tetrahedral sites. For (f) bet M4Ceo, the dopant is also found on distorted tetrahedral sites. For (c) pertaining to small alkali metal ions such as Na, only the tetrahedral sites are occupied. For (h) we see that four Na ions can occupy an octahedral site of this fee lattice.

Blue copper proteins. A typical blue copper redox protein contains a single copper atom in a distorted tetrahedral environment. Copper performs the redox function of the protein by cycling between Cu and Cu. Usually the metal binds to two N atoms and two S atoms through a methionine, a cysteine, and two histidines. An example is plastocyanin, shown in Figure 20-29Z>. As their name implies, these molecules have a beautiful deep blue color that is attributed to photon-induced charge transfer from the sulfur atom of cysteine to the copper cation center. 

Two copper(II) complexes of 2-acetylpyridine thiosemicarbazone, 8, were included in a study of complexes of 2-formylpyridine thiosemicarbazone [169]. [Cu(8-H)OAc] has a magnetic moment consistent with a monomeric copperfll) center and both it and [Cu(8)Cl2] have d,2-y2 ground state ESR spectra (Table 2). A d-d envelope and a magnetic moment of 1.68 B.M. have led others [178] to propose a distorted tetrahedral environment with metal-metal interaction for the brown complex, [Cu(8)Cl2]. 

It should be noted that four-coordinate Cu(II) complexes do not typically favor tetrahedral coordination, so that distortions for this metal center may be more pronounced upon substitution at the 5-position. See Cotton, F. A. Wilkinson, G. Advanced Inorganic Chemistry (5th Ed.) Wiley-Interscience New York, 1988. 

Two equiv. of 6,6-di(cyclopropyl)fulvene react at 60 °C over a period of a week with Ca[N(SiMe3)2]2-(THF)2 bis in THF to yield the metallocene 170. The heteroleptic amido complex 171 is detected as an intermediate with 111 and 13C 1H NMR spectroscopy. A 1 1 reaction of the calcium amide and 170 also produces 171 in solution, an equilibrium involving these three derivatives exists (Equation (30)). The calcocene 170 crystallizes at — 20 °C from THF as colorless cuboids. The metal center is surrounded by the four ligands in a distorted tetrahedral manner, and the cyclopentadienyl group and the propylidene fragment are coplanar with each other.393... 

Metal complexes involving silylene 1 have been described in previous sections for a range of d-block metals. Silylene 1 also reacts with CuI(PPh3)3 and forms CuI(PPh3)2(l). Structural characterization of the latter confirms a distorted tetrahedral coordination environment for the Cu(l) center (Cu-Si = 2.289(4) A).285... 

In the author s own laboratory the Cu(II)-catalyzed hydrolysis of the phosphate ester derived from 2-[4(5)-imidazolyl] phenol recently has been investigated146. The pertinent results are (a) the pre-equilibrium formation of a hydrolytically labile Cu(II)-substrate complex (1 1), (b) the occurrence of catalysis with the free-base form of the imidazolyl and phosphate moieties and (c) the extraordinary rate acceleration at pH 6 (104) relative to the uncatalyzed hydrolysis146. The latter recalls the unusual rate enhancement encountered above with five-membered cyclic phosphates and suggests a mechanism in which the metal ion, at the center of a square planar complex or a distorted tetrahedral complex, might induce strain in the P-O ester bonds (60). viz. 

The mechanism of the regulation of electron transfer in metalloproteins has been investigated 61) and two relevant examples have been discussed in the first one the molecular mechanism controlling the electron transfer reactions is restricted to the immediate chemical environment of the metal center (azurin), while in the second one it involves a conformational transition of the whole quaternary structure of the enzyme. The power of the kinetic approach in detecting significant intermediates was emphasized 6t>. The Cu metal complex site of azurin has a distorted tetrahedral... 

O = 2.17(1) A). In both neodymium ate complexes the ligands adopt a distorted tetrahedral geometry around the metal center. 

Spectroscopic studies of Co (II) derivatives of stellacyanin, plastocyanin, and azurin have established that the charge transfer interpretation is preferred (10, 11). Intense bands (c 2 X 103) that appear to be analogous to the 600-nm system of blue proteins are observed between 300 and 350 nm in the Co (II) derivatives. The shift in band position of about 16 kK [Cu(II) << Co (II)] accords well with expectation for an LMCT transition. The visible and near-infrared absorption, CD, and MCD spectra of Co (II) derivatives of stellacyanin, plastocyanin, and azurin have been interpreted (12) successfully in terms of the d-d transitions expected for distorted tetrahedral metal centers (Table I). Average ligand field parameters are the same for all three Co (II) proteins (Dq = 490, B = 730 cm"1), which strongly suggests that the donor atom... 

---