Pseudotetrahedral geometry

A detailed study involving racemization kinetics and spin states of a series of nine related complexes (507) has been undertaken.1357 In an X-ray crystallographic study of a series of complexes (R Pr1, R2 = Me, R3 = H, R4 = Me, Ph, Bu ) all complexes were found to possess pseudotetrahedral geometry.1358... 

The orange complex 83 (57) exhibits a pseudotetrahedral geometry in the solid state, with the chlorine atoms distorted 49° and 63° away from the bipyridyl-Cu plane (Fig. 8). The d-d transition occurs at 919 nm. It seems likely that the large size of the ligand substituent is responsible for the deviation in the structure of the copper complex. Whether this effect is also responsible for the ease of reduction of the corresponding triflate complex by diazoester is not clear. 

The overlap of carbon p orbitals with arsenic d orbitals is less effective than with the d orbitals of phosphorus, and so the covalent canonical structure is expected to make less of a contribution to the hybrid structure. This has been confirmed in an X-ray study of 2-acetyl-3,4,5-triphenylcyclopentadienetriphenylarsorane.6 Yamamoto and Schmidbaur7 found (13CNMR) that the bonding in arsenic ylides was probably sp3 (cf. phosphorus, which changes from sp3—>sp2), resulting in arsenic pseudotetrahedral geometry (cf. phosphorus ylides, which are planar). 

Relevant structural data for selected mixed phosphine carbonyl complexes are shown in Table 5. In all these complexes the nickel(O) atom is four-coordinate in a pseudotetrahedral geometry with the Ni—CO linkage essentially linear, the Ni—C—O angles being in the range 173-178°. In complex (14) the np3 ligand bonds through the three phosphorus atoms and the r. kel is in a pseudotetrahedral environment.103... 

A number of MO calculations have been performed on carbonyl complexes, with methods ranging from ab initio to DVM-HFS. In any case it was found that both a donation and jr back-donation interactions are important in determining the geometrical structure and physical properties of these complexes. The ab initio calculations of Sakaki et al.110 have shown that the strengthening of jt back-donation is the driving force which stabilizes the pseudotetrahedral geometry vs. the square planar one in [Ni(PR3)2(CO)2] complexes. 

A pseudotetrahedral geometry is imposed by the steric requirements of the tridentate ligand in the complexes [Nilpj]333,334 and [NiSHp3] (79).335,336... 

XHNMR spectra of some dithiocarbazo complexes of zinc, ZnL (HL = R R NHCSSH, have been determined,909 and indicate pseudotetrahedral geometry with chelation by sulfur only. 

In the case of the other hypervalent element compounds, these structural differences are strictly reflected in their terminology. For instance, sulfonium salts such as Me3S+Cl are clearly differentiated from sulfuranes such as Ph2SCl2. The latter is a hypervalent species of decet structure [10-S-4] and pseudotrigonal bipyramid with a linear Cl-S-Cl hypervalent bond however, the former is not a hypervalent compound and has pseudotetrahedral geometry with octet structure [8-S-3]. 

The iodonium ion (type a) generally has a distance of 260-80 pm between iodine and the nearest anion, and may be considered as having pseudotetrahedral geometry about the central iodine atom. 

The structure of the mixed metal complex of Yb and Ni is shown in Fig. 6.27. In the complex Yb has a pseudotetrahedral geometry consisting of two THF molecules and Cp ring centroids of two r)5 -diphenyl phosphino-cyclopentadienides which are also bonded through the phosphorus atom to Ni(CO)2 units [260]. 

M = Tl E = Se) and pseudotetrahedral geometry Ba2AsE2 (E = S, Se) are known. The anions within these phases... 

The heavier alkaline earth metals can also form bis(carbene) adducts, of the type (1)2 MgCp (M = Sr, Ba R = R = Me). An X-ray structure determination of the strontium adduct (16) showed that the strontium center adopts a pseudotetrahedral geometry with two carbenes and two Cp ligands (Sr-C(carbene) 2.854(5) and 2.868(5)A av. 

The crystal structure of ( j -C5H4-tBu)2Zr[( 7 C5H4)2Fe] is shown in Fig. 7-5 [21]. The zirconium atom has a pseudotetrahedral geometry. The cyclopentadienyl rings of the ferrocene unit are eclipsed and deviate somewhat from the parallel disposition (dihedral angle 6°). 

Scheme 7-1 shows a series of ferrocenyl-benzenetricarbonylchromium(i) complexes that have been studied by electrochemistry, in which the chromium atom assumes a pseudotetrahedral geometry. 

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