Square planar compounds

The organometallic complexes of nickel(II) of general formula [NiX(R)(PR3)2] and [NiR2(PR3)2] are invariably diamagnetic square planar compounds like (156)1196 and (157).1200 The cationic complex [Ni(Me)(PMe3)4]BPh4, on the other hand, is five-coordinate.1199... 

Recently, stable organometallic compounds derived from Ni(acac)2 have been described. The square planar compounds [Ni(acac)(PR3)L] (R = Ph, Cy L = Me, Et) were obtained by reacting Ni(acac)2 and PR3 in ether at -20 °C with either diethyl- or dimethyl-aluminum monoethoxide.1546,1547,1577 [Ni(acac)(Me)PPh3] reacts with diphenylacetylene at room temperature, according to equation (183).1578... 

Some square-planar compounds of the type [Ni(AA)2]X2 are reported to undergo reversible thermochroic behavior when heated beyond the temperature range for dehydration. This behavior has been attributed to reversible square-planar to octahedral conversion.87 88 However, the thermochroic behavior of the similar copper compound, [Cu(AA)2](C104)2 (where AA is N,N-di-ethylethylenediamine), does not involve a change in coordination number rather, a change in the in-plane field strength appears to be the cause.89... 

Figure 15 depicts the splittings of the central metal d-orbitals for (a) an octahedral and a distorted octahedral compound and for (b) a square planar and a distorted square planar compound. For this model discussion, one 7i -orbital is introduced to represent the lowest unoccupied molecular orbital (LUMO). In these simplified schemes, other unoccupied 7i -orbitals and occupied 7i-orbitals are neglected for clarity. Thus, all considered electronic states are MLCT states with a central metal d-orbital as highest occupied molecular orbital (HOMO). 

Fig. 15 Schematic splitting of the d-orbitals in an octahedral and a distorted octahedral compound (a) and in a square-planar and a distorted square planar compound (b). In all cases a ti -orbital, representing the LUMO, is displayed in addition (compare [49, 108, 128])...

Similar arguments also hold for 3LC states. In this case, indirect SOCs via Cl are active. Note that the energetic positions of the d-orbitals are important for the size of both Cl and SOC. Thus, smaller ZFSs and lower radiative rates are found for (distorted) square-planar compounds than for comparable (distorted) octahedral compounds. 

In conclusion, under comparable conditions, SOC can become significantly larger for (distorted) octahedral compounds than for (distorted) square planar compounds. This has pronounced consequences on the magnitudes of zero-field splitting and radiative rates of the materials and thus is important for applications of the compounds as emitters in OLEDs. 

Another impetus to mechanistic studies arose from the recognition that compounds of these d ions were those on the energy borderline between stable 18-electron and 16-electron molecules (1) and that the reactions involving transitions between these states are those encountered in catalytic cycles based on these compounds. Nucleophilic ligand substitution, involving association of an entering nucleophile with a square-planar compound, is just one example of the easy 16- 18- 16... 

Despite the richness of this field of study, however, a number of anomalies and some major uncertainties remain. One curious feature, which is probably no more than an historical accident, is that though most studies of nucleophilic ligand substitution have been carried out on complexes of platinum(II) and palladium(II) and few on complexes of rhodium(I) and iridium(I), the reverse distribution is apparent for studies of oxidative additions. The scope for rectifying this imbalance is vast. On the other hand, a fundamental and persistent uncertainty in this field of study concerns the very nature of square-planar compounds in solution. We address this problem in some detail. 

The mechanistic interest in square-planar compounds has meant that the subject has not been short of reviewers, and many excellent articles and chapters have been written. The reader is referred in particular to reviews by Cattalini (4), Tobe (5), Mureinik (6), and Skibsted (7). Other articles are cited at appropriate places. 

Despite the structural uncertainty, the isolation of these compounds, along with mechanistic evidence for such species from reactions we have met and others such as carbonyl insertions at square-planar compounds (163), means that we should reasonably regard such structures as being attainable while keeping an open mind over whether or not they are stabilized by any additional interactions. 

Ligand exchange by dissociative activation now appears to be well established for some 16-electron square-planar compounds, though examples are not common. The detail of the intimate mechanism is crude by comparison with associative reactions, but all the evidence indicates the involvement of T-shaped intermediates. 

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