Square-planar orbitals, coordinate system

Fig. 1.—A molecular orbital coordinate system for a square planar metal complex.

FIGURE 10-12 Coordinate System for Square-Planar Orbitals. 

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). 

Analogous considerations apply for tetracoordinate fragments M(CO)4. Fig. 30 shows some of the possible conformations of these fragments. As before, the directional orbitals that develop for particular values of the angle 6 (refer to Fig. 30) allow prediction of possible interaction with donors or of dimerization. Also, the level shifts for variation of 6 in both cases can be calculated, as well as for the squashing mode rearrangement of a tetrahedral into a square-planar coordination. The qualitative confomiational preferences implied by these patterns have been checked, as for the pentacoordinate case, by comprehensive EHT calculations for all dn systems of all conceivable symmetries. 

Molecular Orbitals for Square Planar Complexes.— Figure 1 shows a square planar complex in a coordinate system with the central atom at the origin, and the four ligands along the x- and y-axes. The orbital transformation scheme in the D4h symmetry is given in Table I. 

A coordinate system for a square-planar complex ML4 (Z>4h symmetry) is displayed in Fig. 8.9.1. The linear combinations of ligand orbitals, matched in symmetry with the metal orbitals, and the molecular orbitals they form, are summarized in Table 8.9.1. A schematic energy level diagram for this type of complexes is given in Fig. 8.9.2. 

Table I compiles typical geometries according to the formal d-orbital electronic configuration of the central metal. Distorted-tetrahedral arrangements, only occur for all <710 systems, whereas for d4, <78, and d9 systems square-planar arrangements occur exclusively. For d5, d6, and d1 systems, square-planar arrangements are most common with some isolated distorted-tetrahedral examples. Under specific conditions, the flat square-planar units can sometimes form strongly joined dimers or trimers (Section III.A.4). In these cases, the coordination geometry about the central atom is best described as square pyramidal.

The restrictions associated with four-coordinate complexes are reversed when the band of d orbitals is filled with metal valence electrons (e.g., d systems). In these situations, ligand field restrictions are encountered from the tetrahedral complex and not the square planar. This departure from the quaUtative picture based on pure d orbitals is primarily due to hybridization factors in these systems. The square planar complex requires an empty d orbital (in the plane) to construct the four hybrid orbitals in the square plane. The [2-f2] transformation from the square planar complex thus returns two valence electrons (formally from a p orbital) to this orbital generating a filled d band in the process. The process proceeds without an orbital crossing. The tetrahedral system, in contrast, starts with a filled d band. The [2- -2] process formally moves a pair of d electrons into a p orbital. This process thus involves an orbital crossing and therefore encounters ligand field restrictions. 

Consider first a system with two electron pairs occupying these orbitals (three valence pairs if a central s orbital is included). Z(a) then becomes 4 (square planar), 3 (tetrahedral) and 4 (tetrahedral) and S (cos a + cos jS) for the SF4 geometry. (For and Cj geometries in general 2(a) is 4 3 cos O and 313 cos 0.) Such a molecule should therefore be square planar. For systems with three p manifold pairs (four valence pairs) e.g. CH4, CX4 the quadratic stabilisation energies for all three systems are equal and we turn to a discussion of the quartic terms to pinpoint the equilibrium geometry. For the square planar structure this term is — 16 7 with a minimum value within the 4 coordinate of — y 7 at an... 

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