Square planar coordination

Evans and co-workers investigated the effect of a number of -symmetric bis(oxazoline) ligands on the copper(II)-catalysed Diels-Alder reaction of an N-acyloxazolidinone with cyclopentadiene. Enantiomeric excesses of up to 99% have been reported (Scheme 3.4). Evans et al." suggested transition state assembly 3.7, with a square planar coordination environment around the central copper ion. In this scheme the dienophile should be coordinated predominantly in an cisoid fashion in... 

The effects of these ligands on the second-order rate constants for the Cu (ligand) catalysed reaction of Ic with 2 are modest In contrast, the effects on IC2 are more pronounced. The aliphatic Oramino acids induce an approximately two-fold reduction of Iv relative to for the Cu" aquo ion. For the square planar coordinated copper ions this effect is expected on the basis of statistics. The bidentate ligands block half the sites on the copper centre. 

Figure 27-10 Anhydrous acetates of Pd" and Pt" (a) trimeric [Pd(02CMe)2h involving square-planar coordinated Pd but no metal-metal bonding (average Pd -Pd = 315pm), and (b) tetrameric [Pt(02CMc)2]4 involving octahedrally coordinated Pt and metal-metal bonds (average Pt-Pi = 249.5 pm). llie four bridging ligands in the Pt,j plane are much more labile than the others.

Mixed 6),Af-donor ligands such as Schiff bases are of interest in that they provide examples not only of square-planar coordination but also, in the solid state, examples of square-pyramidal coordination by dimerization (Fig. 28.6(b)). A similar situation occurs in the bis-dimethylglyoximato complex, which dimerizes by sharing oxygen atoms, though the 4 coplanar donor atoms are all nitrogen atoms. Copper(II) carboxylates are easily obtained by crystallization from aqueous solution or, in the case of the higher carboxylates, by precipitation with the appropriate acid from ethanolic solutions... 

Reaction of [Rh(/z-Cl)(CO)2]2 with sodium pyrazolate leads to 206 (85CJC699). The Rh2N2Cl ring has the envelope conformation. The rhodium atom has distorted square-planar coordination. The molecules in the crystalline lattice form onedimensional stacking units with alternating rhodium atoms in the binuclear units, intermolecularly interacting in a zigzag chain. 

Compound [PtCl( Bu2PCMe2CH2)2l reacts with pyrazole or 3,5-dimethyl-pyrazole in the presence of sodium hydroxide to form 242 (R = H, Me) [84ICA (82)L9]. The chelate ring is not planar in this case, and the trans strucmre of the pyrazolate derivative was demonstrated. Tlie four-coordinated platinum atoms are characterized by a distorted square-planar coordination. 

Rh(Ph2P(CH2)2PPh2)2 CIO4 has essentially square planar coordination of rhodium (Rh-P 2.289-2.313 A) [54],... 

PtS (PdO structure) and PdS (similar) are prepared from M2+(aq.) and H2S or Li2S. They have square planar coordination of M2+ (Figure 3.9). 

However, it seems that these are best viewed as platinum(II) species too, so that two-electron metal-to-ligand transfer has been effected. The structures of Pt(PPh3)2Z (Z = r]2-02, t 2-C3H4, t]2-CS2) (Figure 3.16) all involve square planar coordination as expected for platinum(II) rather than the tetrahedral 4-coordination anticipated for platinum(O). 

Pt(acac)2 has the expected square planar coordination by oxygen (Pt-O 1.979-2.008 A) with bidentate diketonates this has also been confirmed for Pd(PhCOCHCOMe)2, which is obtainable as cis- and trans-isomers that can be crystallized and separated manually (Figure 3.23). 

With their preference for square planar coordination, palladium(II) and platinum(II) are well suited to binding to porphyrins and related N4 donor macrocycles. Therefore, Pd(octaethylporphyrin) is readily synthesized starting from the labile PhCN complex (like the platinum analogue) [92]... 

It has square planar coordination (Pd-N 2.010-2.017 A) similar to the value of 2.009 A in the tetraphenylporphyrin analogue, prepared by a similar route. As with nickel, macrocycle complexes can be made by in situ template... 

Figure 3.67 Part of the hexameric [Pd(SPr )2]6 molecule showing square-planar coordination of palladium. (Reproduced with permission from Acta Crystallogr. Sect. B, 1968, 24, 1623.)...

The 4 1 complex has square planar coordination of platinum (Pt-S 2.317— 2.321 A) similar bond lengths are found in the corresponding complex with 1,4-thioxane [120]. Complexes with thiourea are important in Kurnakov s test (section 3.8.2) Pdtu4Cl2 has square planar coordination (Pd—S 2.33 A). 

Oxidation of [Pt(C6Cl5)4]2- yields the unusual paramagnetic organo-metallic [Pt(C6Cl5)]4 with square planar coordination of platinum... 

The gold(I) atoms have linear coordination (Figure 4.3) and the gold(III) atoms square planar coordination [24]. 

Silver has square planar coordination in Ag[meso-Me6[14]ane](N03)2 (Ag-N 2.16 A) with distant axial oxygens (Ag-O 2.81 A) the complex has... 

The longest established silver(III) complexes are the red to brown bi-guanides, like the ethylene bis(biguanide) shown in Figure 4.14 persulphate oxidation of Ag+ in the presence of this ligand gives a silver(III) complex with essentially square planar coordination. 

Apart from Au(N03)4, relatively few complexes of gold(III), and only those with O-donors, have been examined. Two that demonstrate the preference of gold(III) for square planar coordination are SrAu2(MeCOO)8 and SrAu2(OH)8 in the latter Au(OH)4 has Au-O 1.98 A [117]. 

Square planar coordination is general in these in the tris-complexes Au(S2CNR2)3, it is obtained by two dithiocarbamates being monodentate (the third is, of course, bidentate) [131], Such planar coordination in [Au(S2CNEt2)2]+SbF6 involves Au—S distances of 2.316-2.330 A [132]. 

Neutral, diamagnetic M(//) complexes with the formula Ni(R2 itc)2 are found (2). Stmctural studies for a great variety of R groups (88-93) showed a square planar coordination geometry as is expected for a four-coordinated metal in a tf configuration. 

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