Pseudotetrahedral derivatives

Tetrakis-Cp uranium complexes are readily prepared via metathesis of UCl and KCp in refluxing benzene. These complexes are a relatively rare example of a pseudotetrahedral complex with four Tj -Cp rings, (rj-ring) (25). The Cp derivative has been shown to react with CO and CO2 to give acyl and carboxylato complexes. This complex also reacts with alkyl haUdes to afford the U(IV) complex, Cp UX (X = halide). 

Since the ground state of tetrahedral complexes is orbitally degenerate (37i), it can be expected that the shifts observed in NMR spectra of pseudotetrahedral complexes are due to both contact and pseudocontact contributions. Kurland and McGarvey436 have derived the equations for calculating contact and pseudocontact terms in complexes with nearly orbitally degenerate ground states. Calculations of NMR shifts in pseudotetrahedral complexes are reported in ref. 437. Selected examples of pseudotetrahedral complexes whose NMR spectra have been studied are reported in Table 28 and ref. 455. 

In general, substituents in 2,9-positions of phen and 6,6 -positions of bipy prevent the formation of tris-chelate complexes due to the steric hindrance of the ligands and consequently the mono- and bis-chelate complexes become preferred. The two complexes [NiX2(N—N)] (102 X = C1, Br) formed by 2,9-dimethyl-l,10-phenanthroline have been found to be square pyramidal dinuclear with both bridging and terminal halides,848 while the iodo derivative [NiI2(N—N)] (103) is monomeric and pseudotetrahedral.847... 

The nitrosyl complexes of nickel(II) are scarce and less studied than those of nickel(0) (see Section 50.2.5.2) even though they have been known for nearly a century. Selected examples of nitrosyl nickel(U) complexes are reported in Table 58. As early as 1891 it was reported by Berthelot1117 that Ni(O0)4 reacts with gaseous NO giving a blue compound which was later characterized as a pseudotetrahedral complex of nickel(U) having the formula [Ni(OH)3(NO)].1118 This compound is paramagnetic and is formed only if traces of water are present in the reactants. Using a methanolic solution of Ni(CO)4 a methoxo derivative is formed. 

Benzoxazole (L) derivatives of the stoichiometry M(XCN)2L2 (M = Zn or Cd X = S or Se) and Cd(NCSe)L have been prepared the former are pseudotetrahedral while the latter is polymeric. The ligand is N-bonded in all cases.460 Benzoxazole-2-thione (L) forms the similar MX2L2 complexes (M = Zn, Cd, Hg X = Cl, Br, I) and CdI2L coordination is through nitrogen, and not sulfur or oxygen.461... 

Historically, bis(aminotroponeiminato) nickel(II) complexes have been veiy instructive. The compounds are either pseudotetrahedral or display a tetrahedral-planar equilibrium. The ligands contain seven-membered rings showing alternation of proton shifts and spin densities (Table 2.5). The interest lies in the variety of R derivatives which show how spin density can be transmitted through it bonds, whereas it cannot be transmitted through sp3 carbons or through ethereal oxygen atoms [48,49]. 

In the case of cobalt substituted Zn-fingers [102], the differences between the chemical shifts for corresponding resonances in the Co(II) and Zn(II) complexes allow the determination of the orientation and anisotropy of the magnetic susceptibility tensor [103]. Similar studies are available for pseudotetrahedral Co(II) in the zinc site of superoxide dismutase [104] and five coordinated carbonic anhydrase derivatives [105]. 

Pseudotetrahedral complexes of Cu1, [Cu(bpy)2]BF4 and similar derivatives with substituted dipyridyl ligands all gave intact cations in their ES mass spectra [37,38]. In an extension of this chemistry, bipyridyl groups were introduced into amino acid residues and, after reaction with Cu1, the intact cations of the copper(I) complexes of these species were observed [37]. The analogous [Cu(phen)2]+ ion and others derived from substituted phenanthrolines have also been observed [39]. For both the [Cu(bpy)2]+ and [Cu(phen)2] + species, collisional activation led to loss of one ligand. 

As noted earlier, the chloride ligand in Cp3 And (An = Th, U, Np) is labile and this feature has facilitated the synthesis of a great many tris(cyclopentadienyl)actinide(IV) alkyl derivatives by metathesis with alkyllithium or Grignard reagents. The properties and reactivity of these complexes have been extensively studied and reviewed. They exhibit remarkable thermal stability. As members of the general class of CpsAnX complexes, they adopt the ubiquitous pseudotetrahedral coordination geometry that was illustrated earlier in (2). 

Pseudotetrahedral cyclopentadienyl derivatives that retain the metal-core bonding scheme commonly feature the C03CP3 moiety. Since each CpCo unit contributes one electron fewer than does (CO)3Co, stability requires that the apical group should be a six-electron ligand. Complexes that meet this requirement are of composition CP3C03 (arene), and are prepared thermally from CpCo(C2H4)2 (Section 5.1.2). The synthesis is only successful if the arene bears a vinyl group. ... 

Only a handful of pseudotetrahedral or octahedral oxosulfido-Mo(VI) and -Mo(V) complexes are known, and their chemistry is described in Section 11. Mononuclear sulfido-Mo(V) complexes are unstable due to their susceptibility to redox, polynucleation, and hydrolysis reactions. Trispyrazolylborate derivatives such as sfructuraUy characterized Tp Mo SX2 (X = Cl, OPh derivative X2 = bdt, catecholate) are most commonly encountered. The Tp complexes exhibit short Mo=S bonds (ca. 2.13 A), intense S(ls)- - n (Mo=S) XAS transitions characteristic of terminal thio ligands, and EPR iso see g-Factor) values substantially lower than their 0x0 analogues. ... 

Ln[CH(SiMe3)2]3CI Li(thf)4 19). The ytterbium derivative of the latter complex was crystallographically shown to have a pseudotetrahedral arrangement of ligands around the metal with Yb—C bond lengths of 2.37-2.39 A. 

Mosbach prepared imprinted polymers with not only a more specific substrate binding but with a true catalytic turnover [148]. Certain N-protected amino acid derivatives formed pseudotetrahedral complexes in solution in the presence of Co and two molecules of 5-vinylimidazole. These were polymerised in the presence of divinylbenzene and the metal-ion and amino acids eluted together afterwards. Hydrolysis of the corresponding 4-nitrophenylester of the amino acid template showed an increase in catalytic activity (factor 2-4) compared with a blank prepared without the template. The polymer also exhibits a clear preference for the activated ester of the template, which suggests that hydrolysis indeed takes place inside the cavities. Two years later the scope was further broadened by the preparation of the first polymer imprinted with a transition state analogue. 

Optically active organometallic compounds, especially pseudotetrahedral half-sandwich complexes of the type Cp(OC)(Ph3P)Fe-R with iron as the center of chirality have been extensively investigated in the past. However, synthetic utilization of stereocontrol by the chiral iron has been limited to systems with a-bonded carbon ligands [2]. In contrast, silicon-iron complexes have not yet found analogous application. In context with our studies concerning metallo-silanols we have established simple routes to isolate diastereomerically pure derivatives with a chiral iron fragment. 

This first report of the correctly assigned pseudotetrahedral tungsten derivatives of this complex (Fig. 17), suggested that the alkyne was doubly... 

Finally, a pseudotetrahedral complex more closely resembling the copper site in blue copper proteins, including the presence of two cysteine groups, can be achieved by using a cysteine derivative of ethylenediamine, [HSCHjCHfCOjCHjJNHCHjlj. It is a softer, polydentate (though not macrocydic) ligand and will displace N4 ... 

Fitting of the data for pseudotetrahedral complexes shows that they have Ts of 10 s, whereas five-coordinate complexes have a shorter Tj, on the order of 10 s. The latter derivatives also have exchangeable protons that could correspond to a water molecule in the coordination sphere, whereas the former do not. The Tj values are thus proposed as indicators of the coordination number in low-symmetry, four- and five-coordinate cobalt complexes. The shorter electronic relaxation times are related to low-lying excited states, which, independently of the particular mechanism, favor electron relaxation. " ... 

Many (although not all) spectroscopic data on metal-substituted derivatives and their binary and ternary complexes have also been interpreted as indicative of a four-coordinate metal.Even nickel(II) and copper(II), which have little tendency to adapt to a pseudotetrahedral ligand environment, do so in LADH, the electronic structure of the latter resembling that of blue proteins (Figure 2.36). ... 

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