Square complexes

If the Hermetian conjugate of a square complex matrix is equal to its inverse. 

Geometric isomerism can occur with any square complex of the type Mabcd, Ma2bc, or Ma2b2, where M refers to the central metal and a, b, c, and d are different ligands. Conversely, geometric isomerism cannot occur with a square complex of the type Ma4 or Ma3b. Thus there are two different square complexes with the formula Pt(NH3)2ClBr but only one with the formula Pt(NH3)3Cl+. 

Figure 2.15 Potential-energy curves for linear Li+ CO (circles) andLi+ OC (squares) complexes, compared with the classical ion-dipole estimate (dotted line).

Examples of coordination complexes of pyridine with metals are legion and only a few can be mentioned here. Pyridine is a ligand in square complexes of gold (AuEt2Br-py) and copper (Cupy2-Cl2). The cobalt complex CoCLrpy2 has an octahedral structure. Nickel and platinum can coordinate with four pyridine molecules (30). 

Since a direct displacement process characteristic of reactions of square complexes seems unlikely, it would appear that in such sterically-hindered systems the reaction proceeds by a dissociative process similar to that found for six-coordinated... 

One of the earliest studies of the kinetics of substitution reactions of square planar complexes is that of the Cl exchange of [AuClJ- (22). A two-term rate law was found for the exchange rate and it was suggested that this may prove to be general behavior for square complexes. 

Divalent nickel forms two main types of complexes. The first consists of complexes of the spin-free ( ionic or outer orbital) octahedral type (see also Ligand for their discussion) in which the ligands are principally H2O, NH3, and various amines such as ethylenediamine and its derivates, e.g., Ni(H20>62+. Ni(NH3)e2+, Ni(en)62+. These complexes usually have colors toward the high-frequency side of the spectrum, i.e., violet, blue, and green. The other class consists of tetracovalent square complexes with ligands such as CN, the dioximes and their derivatives, and other chelates, which usually have colors on the low frequency side of the spectrum, i.e., red. orange, and yellow. The structure of the nickel-climethylglyoxime complex is... 

Fujita M, Tominaga M, Hori A, Therrien B (2005) Coordination assemblies from a Pd(II)-comered square complex. Acc Chem Res 38 371-380... 

On the other hand, octahedral complexes of Ni(XI) and Cu(II) should be relatively unstable, since in such complexes electrons would have to occupy the higher-energy d orbitals. Indeed, octahedral complexes of Ni(II) and Cu(II) are rare those that exist are thought to be irregular octahedra with two of the groups (tram to each other) lying significantly farther from the central atom than do the other four. For these two metal ions, square complexes are by far the more usual. 

Mechanistic investigations with square complexes have centered primarily around the complexes of Pt(II), for a large number of such complexes have been prepared and characterized and many react at convenient rates at room temperature. The complexes of Pd(II) and Au(III), also square, react more rapidly by several powers of ten, and those of Ni(II) react more rapidly still. Kinetic studies in the Ni(II) series generally require special techniques. 

A number of workers prefer to regard square complexes in solution as octahedral complexes, with two solvent molecules lying above and below the plane of the four remaining ligands. On this basis (Fig. 23-5) a reac-... 

Today, it is recognized that although tetrahedral complexes of nickel do exist (for example, complex II), they are rare. Many of the blue paramagnetic complexes are now known to have six, rather than four, coordination positions, with two of these positions occupied by solvent molecules or anions (for example Ni(en)2(SCN)2). Moreover, ligand field theory tells us that square complexes of nickel may have either zero or two unpaired electrons. 

On this basis, the conversion of a yellow diamagnetic Ni(II) complex to a blue paramagnetic complex on heating results from excitation of one of the electrons in the dxlf orbital to the dx orbital. Such a transition seems more likely (particularly in the solid state) than the reorganization of a square complex to a tetrahedral one. Since both classes of complex are important for 4-covalent nickel, and since transitions between the two types are often observed, we may conclude that for Ni(II), the dxy and d v levels lie close together. This is not the case for the complexes... 

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