Square planar species

In a square planar species such as [ICLt] or [PtC ] (1-22), the four Cl atoms are equivalent. Similarly, in [PtCl3(PMe3)] (1.23), there is only one possible arrangement of the groups around the square planar Pt(II) centre. (The use of arrows or lines to depict bonds in coordination compounds is discussed in Section 6.11.) 

If the presence of a lone pair of electrons influences the shape of a molecule or ion, the lone pair is stereochemically active. If it has no effect, the lone pair is stereochemically inactive. The tendency for the pair of valence s electrons to adopt a non-bonding role in a molecule or ion is termed the stereochemical inert pair effect. 

Square planar species of the general form EX2Y2 or EX2YZ may possess cis- and trawi-isomers. 

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Figure 3.81 Pathways for substitution of a square planar species PtL3X.

The nickel(II) dithiocarbamate complexes are neutral, water-insoluble, usually square-planar, species, and they have been studied extensively by a range of physical techniques. The usual methods for the synthesis of dithiocarbamate complexes have been employed in the case of Ni(II), Pd(II), and Pt(II). In addition, McCormick and co-workers (330,332) found that CS2 inserted into the Ni-N bonds of [Ni(aziri-dine)4P+, [Nilaziridinelgf, and [Ni(2-methylaziridine)4] to afford dithiocarbamate complexes. The diamagnetic products are probably planar, but they have properties typical of dithiocarbamate complexes, and IR- and electronic-spectral measurements suggested that they may be examples of N,S-, rather than S,S-, bonded dithiocarbamates. The S,S-bonded complexes are however, obtained, by a slow rearrangement in methanol. The optically active lV-alkyl-iV(a-phenethyl)dithio-carbamates of Ni(II), Pd(II), and Cu(II) (XXIV) have been synthesized, and the optical activity was found to be related to the anisotropy of the charge-transfer transitions (332). 

Ni has a smaller radius in square planar geometry than in tetrahedral, which is assumed to result from the non-Aufbau electron filling in ds square planar species.322 The well-known thermal octahedral square planar transformation of Ni11 coordination compounds has been... 

The thermodynamic data of Eq. (11) for the various Ni(II) macrocyclic complexes are summarized in Table IV. Since the octahedral species should have longer Ni—N bond distances than the square-planar species, the complexes with flexible macrocyclic ligands have larger values. For example, the formation constants of octahedral species for... 

On account of their diamagnetism and spectral properties the [NiX2(dpe)2] complexes have a substantial square planar structure in the solid state, possibly with two weakly interacting anions in axial positions, as found in Nil2(diars)2 (vide infra). In solution either five-coordinate or square planar species are formed.1277 3283,1347 The ligand tep (II) easily forms diamagnetic five-coordinate complexes [NiX(tep)2]Y in the solid state and in solution as well.1276,1281,1282 The structure of [NiI(tep)2]I is essentially square pyramidal.1281... 

Complexes of several cyclic tetramines with Ni11 exist in aqueous solution as an equilibrium between singlet ground state, square planar species and triplet ground state, diaquo species (see Table 2).32... 

Valuefs) for singlet ground state, square planar species Ai/e 52.3. k 1,4,8.11-Tetraazacyclotetradecane, cyclam,... 

In the solid state, the metal atoms in bis(salicylaIdoximato)copper(II) show two additional contacts with the oxime oxygens of adjacent molecules, resulting in a distorted octahedral structure. However, the axial Cu—O distance (2.66 A) is much longer than the metal—ligand distances in the square-planar array (Cu—O, 1.92 A and Cu—N, 1.94 A).154 Studies by ESR of copper(II) extracts of the commercial reagent SME 529 (14 R = Me, R = C9H)9) have shown that the copper complex exists as a square-planar species in hydrocarbon solutions, but that five-coordinate adducts are formed in the presence of ammonia or pyridine.155... 

In coordination chemistry, many square planar species are known (for example, PtCl4 ). For a square planar molecule, use the appropriate character table to determine the types of hybridization possible for a metal surrounded in a square planar fashion by four ligands (consider hybrids used in a bonding only). 

Sixteen-electron square-planar species are most commonly encountered for... 

Under certain conditions, ethylenediamine itself may form square planar complexes with nickel(II). Sone and Kato (89) reported that yellow Ni(en)2" is formed when [Ni(en)2(H20)2]" is heated in alcohol. The bis-ethylenediaminenickel(II) complex, Ni(en)2(AgIBr)2, and the corresponding platinum(II) and palladium(II) derivatives are isomorphous, indicating square planar nickel in this complex (58), The anions may be changed to Cul2 , Is", Pbls , and B.glr without destroying the planarity of nickel. Cobalt (II) and copper(II) also form square planar complexes of this type. These square planar species appear to be stable only in the solid state for they decompose when dissolved in water (58). 

The two square planar species of line 2 are diastereomers. They contain the same optically active chelating phosphine chiraphos, but the rhodium atom is coordinated to different sides of the prochiral olefin (re/si sides). The two diastereomers of line 2 are rapidly interconverting. In this equilibrium the isomer shown on the left-hand side (si-coordination of the olefin) is the minor isomer and the isomer shown on the right-hand side (re-coordination of the olefin) is the major isomer [91, 92]. 

The next step is the oxidative addition of hydrogen, converting the square planar diastereomers of line 2 into the octahedral dihydrides of line 3 [93]. In the present system this reaction is the rate-determining step. The fast step following is the insertion of the coordinated olefin into one of the Rh-H bonds, giving rise to the two diastereomeric (T-alkyl complexes of line 4. By reductive elimination they generate the enantiomeric forms of the product, regenerating the catalytically active square planar species, which reenters the catalytic cycle. 

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