Square-planar fragments

Octahedral fragments of formula ML (where M has a configuration) are isolobal with square-planar fragments of formula ML 2 (where M has a d configuration and L = 2-electron donor) ... 

FIGURE 15-4 Comparison of Square-planar Fragments with Octahedral Fragments. 

We shall consider the tt and tt orbitals on the ethylene ligands in each conformation, as they interact with the orbitals of the square-planar fragment in which the three nonbonding d orbitals, xy,... 

The crystal structure of a CODH/ACS enzyme was reported only in 2002.43,44 It reveals a trio of Fe, Ni, and Cu at the active site (6). The Cu is linked to the Ni atom through two cysteine-S, the Ni being square planar with two terminal amide ligands. Planarity and amide coordination bear some resemblance to the Ni porphinoid in MCR. A two-metal ion mechanism is likely for acetyl CoA synthesis, in which a Ni-bound methyl group attacks an adjacent Cu—CO fragment with formation of a Cu-acyl intermediate. A methylnickel species in CODH/ACS has been identified by resonance Raman spectroscopy.45... 

Figure 17 The (chol)-P- -P(chol) fragment of the square planar traw -[PtI2(3-/ -PMe2chol)2] complex adopts...

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. 

It results that in the absence of any fragmentation of the original frame, the process [Pt(C6Cl5)4]-/0 is in principle quasireversible. The large peak-to-peak separation (A/sp = 1.9 V, at 0.2 V s-J) evidently reflects the high energy barrier required to pass from square planar tetracoordinate-Pt(III) to octahedral hexacoordinate-Pt(IV), which slows down the rate of the heterogeneous electron transfer. 

Haegele et al. (269) have used exact isotope masses and isotope abundances together in determining the detailed fragmentation patterns of square planar rhodium (I) -diketonate complexes. They found that some species postulated by other workers were in error. High resolution is needed to distinguish the 28 mass units for loss of CO (27.9949) from C2H4 (28.0313) (269) or the 69 mass units for PF2 (68.9906) from CFa (68.9952) (90). 

The LUMO in d pentatetraenylidene complexes is predominantly localized on the odd carbon atoms and to a lesser extent on the metal. The coefficients on Cl and C3 are very similar, independent of the metal-ligand fragment and the terminal substituent. The coefficient at C5 is somewhat larger. In square-planar d rhodium and iridium complexes the coefficient at the metal is comparable to that on C5 and is larger than those on Cl and C3. Thus, a nucleophilic attack at the metal of d complexes has also to be taken into account. 

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