Five coordinate Ni complexes

A series of air-sensitive trigonal bipyramidal and trigonal pyramidal Ni complexes, [Ni(PR3)4]+, [NiX(PR3)3], [NiX(AsR3)3] (X = halide), and [Ni(tripod)]+ and [NiX(tri-pod)] (tripod = face-capping tripodal P, As ligand), have been reported. The methods of synthesis of these species are varied, but essentially are based on the reduction of Ni or oxidation of Ni precursors. A range of four- and five-coordinate Ni complexes have been stmcturally characterized and some of these are illustrated in (42)-(46). 

Burdett has examined the CFSE concept in terms of the AOM. Essentially similar conclusions can be reached concerning the relative stabilities of configurations. The case of Ni (d ) is particularly interesting. This has maximum CFSE for octahedral coordination, but other geometries are competitive for example, Cr is rarely found in any coordination other than octahedral, but four- and five-coordinate Ni complexes are abundant, and octahedral Ni is not conspicuously inert. The removal of two ligands to convert octahedral to square planar will stabilize dj.2 (see also Section 7.2) and it can be seen that the change from Axyf dxzf Ayzf dz y d 2 y2y to dxyf dxzf dy f d 2f will lower the orbital energy and enhance CFSE. Octahedral to square equilibria of the type... 

Five-coordinate Ni111 complexes (89) have been prepared by oxidation of the square planar Ni11 precursor complexes [Ni(L)X] with either X2 or CuX2, and the crystal structure of the iodo derivative has been determined. The geometry at Ni is best described as square pyramidal, with the Ni atom displaced approximately 0.34 A out of the basal plane towards the apical I atom. EPR confirms the Ni111 oxidation state, in which the unpaired electron of the low-spin d1 system is situated in the dz2 orbital.308,309 In aqueous solution full dissociation of both X anions occurs, while in acetone solution dissociation is not significant. The redox couple [Nin NCN (H20)]+/ [Ni111 NCN (H20)ra]2+ in water is +0.14V (vs. SCE). 

A for the 5-coordinate species, and 2.01 A for the unligated B. transient excited state. In 5-coordinate complexes the axial ligand-Ni stretching mode has been identified. The mode can also be used to follow coordination changes. Five-coordinate Ni-porphyrin complexes have been observed, so far, only in the Ni-reconstituted proteins. 

The Ni11 (mac-) turns to the five-coordinate Ni(I)-CO species on reaction with CO. Introduction of CO gas into the solution of square-planar Ni(I) species also produces five-coordinate Ni(I)-CO complexes, although they unusually contain 19 valence electrons (66, 135). 

The spectral data of the five-coordinate complexes of N -CO and N -NH—C(OH)CH3 are summarized in Table VIII. The visible band of the five-coordinate Ni -CO complex of the macrocycle that does not... 

The four-coordinate Ni(I) complexes with saturated ligands exhibit anisotropic axial spectra with gg values being greater than g L values. The Ni(II) species of ligand-stabilized radical, which is produced from the reduction of the four-coordinate Ni(II) macrocyclic complexes with conjugated double bonds, exhibit isotropic spectra. The isotropic spectra of Ni11 (mac-) turn to anisotropic axial spectra as the complexes coordinate CO to form five-coordinate Ni(I)-CO adducts (135,137). The anisotropic axial epr spectra of four-coordinate Ni(I) complexes become rhombic as the complexes coordinate extra axial ligand such as CO or acetamide to form five-coordinate Ni(I) species (135, 136). The epr spectral data of some Ni(I) complexes are summarized in Table VII. 

Few X-ray structures have been reported for the square-planar Ni(I) macrocyclic complexes (57,66,134,137), and only one has been reported thus far for the five-coordinate Ni(I) macrocyclic complexes (136). In general, Ni—N bond distances in Ni(I) complexes are anticipated to be much longer than in Ni(II) complexes, since the Ni(I) ion should be larger than the Ni(II) ion in the square-planar geometry, and UV-vis spectra indicate that ligand field strengths of Ni(I) complexes must be significantly weaker than those of Ni(II) complexes. However, X-ray structures of Ni(I) macrocyclic complexes are often inconsistent with such expectations. 

The square-planar Ni(I) complexes form five-coordinate Ni(I) carbonyl complexes when CO gas is introduced into solutions of the complexes, because they contain electron-rich Ni(I) ions capable of it back-donation (66,135). The CO binding constants and carbonyl vibrational frequencies are summarized in Table XI. Because of the back-bonding interaction between the Ni(I) atom and the CO ligand, CO stretching frequencies for the Ni(I) carbonyl complexes decrease as NiL+ becomes a more powerful reductant, which is represented by the Ev2 values (66). 

FIGURE 16.17. The three types of five-coordinate d complexes and their relationship to the Berry pseudorotation mechanism (see Figure 16.9). It appears that five-coordinate complexes of Rh, Ir, and Ni can undergo pseudorotation between a TBP and e-SQP structure. Other five-coordinate complexes of Ni, plus those of Pt and Pd, form f-SQP structures and do not undergo pseudorotation. 

---