Square coordination geometry

Figure 2.15 The molecular structure of the [Cun(16)]2+. The hydrogen atoms of ligand 16 have been omitted. The Cu11 center experiences a rather distorted square coordination geometry. Structure redrawn from data deposited at the Cambridge Crystallographic Data Centre CCDC 118957.

Figure 2.18 A square scheme illustrating the disassembling of the [Cu2(16)2]2 + double helicate complex, following Cu -to-Cu" oxidation, and the consequent assembling of two [Cun(16)]2+ mononuclear complexes, following the Cu"-to-Cu reduction. The process ultimately derives from the geometrical coordinative preferences of the two oxidation states Cu1 prefers a tetrahedral coordination, which can be achieved with the double helicate arrangement Cu11 prefers a square coordination geometry, which is fulfilled by the coordination of a single molecule of 16.

The most common oxidation states, corresponding electronic configurations, and coordination geometries of iridium are +1 (t5 ) usually square plane although some five-coordinate complexes are known, and +3 (t7 ) and +4 (t5 ), both octahedral. Compounds ia every oxidation state between —1 and +6 (<5 ) are known. Iridium compounds are used primarily to model more active rhodium catalysts. 

Since X-ray determinations of structure were too time-consuming to be widely used in the l9-2()s and 1940s and. in addition, square-planar geometry was a comparative rarity, any paramagnetic compound, which on the basis of stoichiometry appeared to be 4-coordinate, was presumed to be tetrahedral. 

M(NO)(OCOCF3)2(PPh3)2. Both these complexes have 5-coordinate geometries with monodentate carboxylates. The rhodium compound has a square pyramidal structure with bent Rh-N-O (122°) but the iridium compound has a tbp structure with straight equatorial Ir-N—O (178°). The position of i/(N—O) reflects this difference (1800 cm-1 (Ir) and 1665 cm-1 (Rh)). 

There is significant metal-metal bonding in the platinum compound, whose geometry involves a square of platinum atoms another important difference is that the coordination geometry is square planar in palladium acetate but octahedral in the platinum analogue. Different oligomers exist in solution, broken down by adduct formation. Palladium(II) acetate may be obtained as brown crystals from the following reaction [65] ... 

In nickel and palladium dithiobenzoato complexes, four-membered chelate rings are formed (366), whereas, in the corresponding phenyl-dithio acetates [M2(S2CCH2Ph)4], the dithio ligands act as bridging groups between the two metal atoms, with the formation of binuclear units (367). The molecular structure of the latter compounds shows that each metal atom is coordinated to four sulfur atoms and to the other metal atoms in a square-pyramidal geometry. Other evidence for... 

If the BF2 groups in Ni(dmg-BF2)2 are substituted by BPh2 units (122), the complex also adopts the saddle-shaped conformation of type D (Fig. 32), in which the two dimethylglyoxime fragments are bent down from the N4 plane with a dihedral angle of about 27° between the two least-squares planes of the dioxime units. The coordination geometry around the nickel ion is distorted square-pyramidal, but there are no intermolecular Ni Ni interactions [167]. 

Additional results indicate that the formation of (XV) is the result of the fact that S-coordinate Ir(III) complexes prefer a square-pyramidal geometry with the vacant octahedral site trans to the most strongly trani-labilizing ligand (COEt) [R. J. Mawby, private communication (March 1972)]. 

A in the dinuclear complex to 2.00-2.004 A in the oxidative-addition products. The Au atoms have a nearly square-planer coordination geometry. 

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. 

The enantioselectivity of this catalyst, which is prepared as the iodide salt, is somewhat dependent on the anion that is present. If AgSbF6 is used as a cocatalyst, the iodide is removed by precipitation and the e.e. increases from 81 to 91%. These results indicate that the absence of a coordinating anion improved enantioselectivity. Entry 2 shows the extensively investigated f-BuBOX ligand with an A-acryloylthiazolidinone dienophile. With Cu2+ as the metal, the coordination geometry is square planar. The complex exposes the re face of the dienophile. 

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