Pseudo-tetrahedral

Cationic cyclopentadienyl complexes are not common in this group, but recent examples whose structures have been determined include [Nb ( j -C5H5)2Cl2]BF4< > and [Nb( j -C5H5)2L2](BF4)2 (L = CNMe and NCMe),< > which have pseudo-tetrahedral symmetry. 

Ru(NO)2(PPh3)2 has a similar electronic structure to the [M(NO)2(PPh3)2]+ (M = Rh, Ir) ions and like them has a pseudo tetrahedral structure with linear Ru-N—O [126]. It also resembles them in its oxidative addition reactions (Figure 1.47). 

Espelid and B0rve [100] have recently explored the structure, stabihty, and vibrational properties of carbonyls formed at low-valent chromium boimd to sibca by means of simple cluster models and density fimctional theory (DFT) [101]. These models, although reasonable, do not take into consideration the structural situations discussed before but they are a useful basis for discussion. They foimd that the pseudo-tetrahedral mononuclear Cr(II) site is characterized by the highest coordination energy toward CO. 

The role of coordinated ethylene is evidenced by the recent ab initio calculation performed by Espelid and Borve [121-123], who have shown that ethylene may coordinate in two different ways to the reduced Cr(II) species, either as a molecular complex or covalently bound to chromium. At longer Cr-C distances (2.36-2.38 A) an ethylene-chromium zr-complex forms, in which the four d electrons of chromium remain high-spin coupled and the coordination interaction is characterized by donation from ethylene to chromium. Cr(II) species in a pseudo-tetrahedral geometry may adsorb up to two equivalents of ethylene. In the case of a pseudo-octahedral Cr(II) site a third ethylene molecule can also be present. The monoethylene complex on the pseudo-tetrahedral Cr(II) site was also found to undergo a transformation to covalently bound complex, characterized by shorter Cr-C distances (about... 

X-ray diffraction studies on several forms of the enzyme have demonstrated that the active site is composed of a pseudo-tetrahedral zinc center coordinated to three histidine imidazole groups and either a water molecule [(His)3Zn-OH2]2+ (His = histidine), or a hydroxide anion [(His)3Zn-OH] +, depending upon pH (156,157). On the basis of mechanistic studies, a number of details of the catalytic cycle for carbonic anhydrase have been elucidated, as summarized in Scheme 22... 

The bi- and tridentate phosphines dppp (l,3-bis(diphenylphosphino)propane) and dppep (bis (2-diphenylphosphinoethyl)phenylphosphine) have been complexed with Co and their divalent four- and five-coordinated thiophenolate complexes Co(dppp)(SPh)2 and Co(dppep)(SPh)2 have been isolated and structurally characterized.379 Somewhat related to dppp is the bidentate silane Ph2PCH2SiMe2CH2PPh2, which forms high-spin, pseudo-tetrahedral dihalocobalt(II) complexes.380... 

The zinc atom is pseudo-tetrahedrally surrounded by the cyclopentadienyl and methyl groups and the chelating TMEDA ligand, while the dimethylpyrrole pendant is not coordinated to the metal. The zinc-methyl bond (1.985(2) A) and the (vO-bound cyclopentadienyl ligand (2.185(2) A) are both slightly elongated, possibly due to the steric crowding about zinc. 

The complex has crystallographic ///-symmetry, the mirror plane bisecting the unique benzyl group, the nitrogen atom to which it is attached, and the ethylzinc moiety. The pseudo-tetrahedral zinc atom has a short (1.930(4) A) zinc-ethyl bond, but comparatively long (2.230(2) A) nitrogen-zinc donor bonds. 

The coordination geometry about zinc is pseudo-tetrahedral with long zinc-nitrogen (av. 2.139(3) A), zinc-carbon (1.997(4) A), and zinc-oxygen (1.956(2) A) bonds. 

The two chemically different zinc atoms show decidedly different coordination environments. Thus, the inorganic zinc atom (Znl) has an octahedral environment and is surrounded by four oxygen and two nitrogen atoms, while the organometallic zinc atom (Zn2) is in the pseudo-tetrahedral environment of one ethyl group and three oxygen atoms. 

The free selenite ion has a pyramidal shape (C3v symmetry) owing to the lone electron pair at the selenium atom. Thus, the Se032- ion can be treated as a pseudo-tetrahedral anion and the lone electron pair often acts as an invisible ligand within the crystal structures of selenites. This observation is called the stereochemical activity of the lone electron pair and it will turn out as one of the... 

Let us start our examination with the prototypical blue protein plastocyanin, found in the thylacoid membrane of chloroplasts, where it acts as an electron carrier in photosynthesis (see Figure 1). As Figure 30 illustrates, the active site of plastocyanin is formed of a Cu(II) ion (pseudo)tetrahedrally coordinated to two histidine nitrogen atoms and... 

The molecular structure of [(TIMEN )Co(N(p-PhOMe))] (3 -Co (N(p-PhOMe)), presented in Fig. 18, shows a pseudo-tetrahedral geometry featuring a short Co-Niinido bond (1.675 (2)A) 36). This short distance, together with an almost linear Co-N-C angle of 168.6(2)°, indicates strong multiple bond character within the Co-NAr entity. According to DFT studies, this bond is most accurately formulated as a double bond 36). 

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