Square-planar system

To this point the complexes considered have shared the coordination number six and approximate octahedral geometry. It has been argued that they also share the dissociative reaction mode. There are examples of reactions both with and without intermediates of reduced (that is, 5) coordination, but the insensitivity to entering ligands is a consistent feature. It will be interesting, shortly, to see if the dissociative pattern persists in more or less organometallic octahedral systems but first we shall give some attention to the non-labile square planar systems. 

In (393), the average Ni—Se distance is 231.7(2) pm.1085 Simple tetrahedral [Ni(SePh)4]2 has been prepared by reaction of NiCl2-4H20 with LiSePh.1087,1088 An excess of selenolate has to be used to prevent formation of [Ni(SePh)2]8. The PPh4+ salt is stable in air for several hours. Its average Ni—Se bond lengths (240.1(3) pm) are longer than in the homoleptic square planar systems. 

In square-planar systems, however, the distinction between axial and equatorial substituent positions does not appear... 

Using the knowledge that sterically blocking the axial positions in square planar systems effectively prevents an octahedral configuration from forming, nickel(II) and copper(II) complexes of a secondary mediumring diamine, 1,5-diazacyclooctane (daco), were synthesized (Figure 9) (67). 

The terms cis and trans are used commonly as prefixes to distinguish between stereoisomers in square planar systems of the form [Ma2b2], where M is the central atom, and a and b are different types of donor atom. Similar donor atoms occupy coordination sites adjacent to one another in the cis isomer, and opposite to one another in the trans isomer. The cis-trans terminology is not adequate to distinguish between the three isomers of a square planar coordination entity [Mabcd], but could be used, in principle, for an [Ma2bc] system (where the terms cis and trans would refer to the relative locations of the similar donor atoms). This latter use is not recommended. 

The configuration index for a square planar system is placed after the polyhedral symbol (SP-4). It is the single digit which is the priority number for the ligating atom trans to the ligating atom of priority number 1, i.e. the priority number of the ligating atom trans to the most preferred ligating atom. 

Complexes with a configuration often form square planar complexes (see Section 20.3), especially when there is a large crystal field Rh(I), Ir(I), Pt(II), Pd(II), Au(III). However, 4-coordinate complexes of Ni(II) may be tetrahedral or square planar. The majority of kinetic work on square planar systems has been carried out on Pt(II) complexes because the rate of ligand substitution is conveniently slow. Although data for Pd(II) and Au(III) complexes indicate similarity between their substitution mechanisms and those of Pt(II) complexes, one cannot justifiably assume a similarity in kinetics among a series of structurally related complexes undergoing similar substitutions. 

In square-planar systems, however, the distinction between axial and equatorial substituent positions does not appear to be so marked — see B. Bosnich and E. A. Sullivan, Inorg. Chem., 1975, 14, 2768. 

The square planar system was different (Figure 16.1) in that oncp orbital at the metal, 16.4, found no symmetry match. There are four metal orbitals primarily of d character at moderate energies, and 16.4, which lies at an appreciably higher energy. It is unreasonable to expect that two electrons should be placed in 16.4 and therefore, stable square planar ML4 complexes have 16 valence electrons. A trigonal ML3 complex will also have one empty metal p orbital. 16.5, and a stable complex will thus be of the 16-electron type. Linear ML compounds have two nonbonding p AOs, 16.6, so here a 14-electron complex will be stable. 

Oxidative-addition reactions. In Section 4.10, oxidative addition reactions were introduced with examples in which a halogen, Br2, was added to square planar systems. In such reactions, both oxidation and addition occur. Important reactions of this type in organometallic chemistry involve addition of molecules such as H2 and organic halides which are not normally considered to be oxidizing agents, reactions (19) and (20). 

Reaction Mechanisms of Inorganic and Organometallic Systems 4.4 ISOMERISM IN Square-Planar Systems... 

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