Nickel complexes pseudotetrahedral

The formation in solution of nickel complexes with TV-substituted ethylenediamines has been studied over a long period by many authors.707 Solutions of nickel(II) complexes with TV-substituted diamines often exhibit equilibria between pseudotetrahedral and pseudo-octahedral species. These equilibria are displaced towards the pseudotetrahedral species when the temperature increases. Complexes Ni(TV,TV-Et2en)2X2691 with poorly coordinating anions are thermochromic. This behaviour has been investigated by means of calorimetric and NMR studies.708,709... 

The tetrahedral coordination of Ni(PF3)4 has been confirmed by two independent electron diffraction studies of the compound in the gaseous state,82,83 giving the following molecular parameters Ni—P = 210-211 pm, ZPNiP = 109° (not refined). Tetrahedral coordination around nickel(O) has also been found in tetrakis(2-thienyldifluorophosphine)nickel, Ni PF2(C4H3S) 4, in the solid state. Relevant molecular distances and angles are Ni—P 209 pin, Z.PNiP 106-1110.84 The Ni—P bond distances found in these two complexes are noticeably shorter than those found in other pseudotetrahedral complexes of nickel(O) with... 

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... 

Pseudotetrahedral nickel nitrosyl complexes [NiLNO] have also been reported with the novel ligands dimethyl(3,5-dimethyl-l-pyrazolyl)(2-thioethoxyethoxy)gallato - and (dimethyl(3,5-dimethyl-l-pyrazolyl)(2-dimethylaminoethoxy)gallato .137,138 -... 

Relevant structural features of the S02 complexes are collected in Table 16 together with the IR absorption frequency of SO2. In complex (63) the nickel atom is in a distorted trigonal bipyramidal environment,250 with a bent coordinated S02 molecule, while in [Ni(p3)(S02)] (64) S02 is coplanar with nickel which achieves a distorted pseudotetrahedral structure. 

Typical spectra of pseudotetrahedral nickel(II) complexes are shown in Figure 11,... 

The nature of the Niu—P bond in pseudotetrahedral phosphine complexes has received much attention. With the AOM Gerloch et al. showed that phosphine bonds are characterized by a large o basicity and large it acidity corresponding to a back-donation of electrons from the nickel to the phosphorus atom in both mono and bis phosphine complexes.549,550... 

Table 30 Magnetic Moments and Spectral Parameters for some Pseudotetrahedral Nickel(II) Complexes ...

Selected nickel(II) complexes with TV-substituted diamines are also shown in Table 41. It is usually found that as the number and size of the substituents increase, the number of the coordinated diamines decreases, as does the stability of the complexes which are, in general, sensitive to moisture. The complexes are soluble in dry solvents without dissociation or decomposition. Pseudotetrahedral coordination is stabilized by increasing the steric hindrance on the donor atoms. In the series of tetrasubstituted diamines the pseudotetrahedral species are stabilized in the order Me4en < Me4pn < Me4tmd and NCS < Cl < Br < I (Table 41).646... 

The nickel halide complexes of Schiff bases (113) and (114) obtained from benzaldehyde and a number of diamines have the formula NiX2L (X = Cl, Br, I) with pseudotetrahedral structure in the solid state and in solution as well.954... 

Bis(aminotroponeiminato)nickel(II) complexes (124) give rise to square planar pseudotetrahedral equilibria in solution.99 1000 The amount of the paramagnetic pseudo-tetrahedral species increases as the size of the substituents R becomes greater. 

Bis(pyrrole-2-aldiminato)nickel(II) complexes (125) are diamagnetic in the solid state when R = H, Pr, Pr and Et, and paramagnetic pseudotetrahedral when R = Bu. 1001-1003 In solution there exists an equilibrium between square planar and tetrahedral species when R = Pr1, Bus and Bu Such equilibria were also investigated for complexes of the type (126) obtained from the condensation reaction in basic media of o-aminobenzaldehyde and a number of diamines in the presence of nickel(II).1004-1007 Square planar complexes (127)1008,1009 and (128)1010 were obtained with deprotonated pyridinecarboxamide ligands. In these complexes the Ni—N (amide) bond distance (184-187 pm) is shorter than the Ni—N (pyridine) distance (192-195 pm). 

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. 

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]. 

Nickel(II) is a 3d8 ion and has two unpaired electrons (5 = 1) when it is six coordinated or four coordinated pseudotetrahedrally. In the latter configuration, sharp proton NMR signals are obtained. When it is planar four coordinated nickel(II) is always low spin diamagnetic (5 = 0). Five-coordinated nickel(II) complexes can either be high (5 = 1) or low spin (5 = 0) depending on the nature of the donor atoms. 

A change in coordination number from six to four perturbs the electronic relaxation times of nickel(II) complexes, in a sense opposite to that observed in cobalt(II) complexes. Nickel(II) complexes, when six coordinated, have an orbitally nondegenerate ground state in octahedral symmetry, with the first excited state at several thousands wave numbers in pseudotetrahedral symmetry, on the contrary, there are always closely spaced levels originating from a threefold... 

In addition to the complexes listed in Table VII, we have used 13C NMR to determine the structure of the product of the reaction of (CDT)Ni with 2,3-dimethylbutadiene (65). The spectrum (-40°C) consists of six signals—two unprotonated carbon atoms at 102.3 and 101.7 ppm, two methylene carbon atoms at 61.1 and 48.4 ppm, and two methyl carbon atoms at 21.8 and 19.4 ppm. A determination of J(CH) for the methylene C atoms gave values of 156 and 157 Hz (both 3 Hz), clearly indicating that they are both sp2-hybridized and thereby eliminating structures such as 20b. The spectrum is consistent with a pseudotetrahedral arrangement of the two diene molecules about the nickel atom (20a). 

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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