Square planar Co complex

The sterically crowded ligand (106) forms a very rare square-planar Co complex, as shown by an X-ray crystal structure, and a spin triplet (paramagnetic) ground state was also identified in the solid state. The Co complex of (107) catalyzes the epoxidation of norbomene with t-BuOOH or Phi as terminal oxidant, catalysis driven by formation of t-BuOO radicals employing a Co redox process. 

Square-planar Co complexes also give an 5 = /z ground term but, apparently because of low-lying excited /-orbitals, give magnetic moments well above the spin-only value, with behaviour corresponding to equation (75). /ieuis commonly 2.2 BM, but Co(phthalocyanine) is an extreme example, with /[Pg.289]

In their schematic representation (Scheme llA), only two coordination sites are accounted for in the cobalt binding and there are no structural investigations presented to support the notion of well-ordered active sites. In addition, square planar Co" complexes are substitution labile, which is an inherently bad trait for template complexes. Therefore, there is a high degree of uncertainty regarding the composition of the reactive cavitites. 

Simple ligand-field arguments, which will be elaborated when M ions of the Ni, Pd, Pt triad are discussed on p. 1157, indicate that the configuration favours a 4-coordinate, square-planar stereochemistry. In the present group, however, the configuration is associated with a lower oxidation state and the requirements of the 18-electron rule, which favour 5-coordination, arc also to be considered. The upshot is that most Co complexes are 5-coordinate, like [Co(CNR)5j, and square-planar Co is apparently unknown. On the other hand, complexes of Rh and Iri are predominantly square planar, although 5-coordination docs also occur. 

The l,2-bis(dicyclohexylphosphino)ethane (dcpe) bidentate ligand forms the mononuclear, square-planar Ir1 complexes [Ir(cod)(dcpe)lBPh4, [Ir(dcpe)2]BPh4, and JTIrCl(CO)(dcpe)], which have been characterized by IR, ll, and 31 P 1II) NMR spectroscopy. 1 Products of reaction with CO are discussed. 

NIR region (see Chapter 9.13), complexes of 1,2-dithiolene (DT) and related ligands have attracted considerable attention for their (largely cubic) NLO properties. The complex (156) (a.k.a. BDN) is a highly photochemically stable, saturable absorber and has hence found extensive applications in laser Q-switching. The cubic NLO properties of (156) have been studied by DFWM148,403-407 and more recently, Z-scan.408 Time-resolved DFWM has been applied to square planar Co, Ni, Cu, or Pt complexes of 1,2-benzenedithiolate (BDT) or 1,2-aminobenzenethiolate ligands by Lindle and co-workers.409,410... 

Besides, comparisons with other non-macromolecular gelling systems are in progress. Specially, we can co.mpare with a square planar copper complex, which aggregates in linear chains to gelify the cyclohexane (l ). It is immediatly noticed that characteristic times of the aggregation kinetics are correlated to the complexity of the molecular aggregation mechanism involved. 

The active species in the catalytic cycles are square-planar Pd complexes of the formula [Pd (X)(S)(L-L)]Y where L-L is a chelating ligand with the same or different donor atoms among P, N, O and S X is the growing polyketone chain or hydride S may be a solvent molecule, a co-monomer, or a keto group from the chain. Finally, Y is a counter-anion of weak nucleophilicity in order to avoid competition with the co-monomer for coordination to palladium (Scheme 7.1). 

The mechanism by which the d-d transitions gain intensity still remains to be determined. For octahedral and square planar complexes which have a center of symmetry, the transitions are partly inhibited. Slightly disorted octahedral and square planar metal complexes may have a fractional part of a d-d transition allowed, and this static distortion mechanism may be responsible for some intensity in many cases (I, 2). The fact that when Co(acac)3 catalyst was utilized in the oxidation, its absorption wavelength remained unchanged and its coefficient increased,... 

Figure 3.10 Examples of a CO insertion and b CH3 migration, in a square-planar Pd complex with a P—N chelating ligand. Note that the migration mechanism retains the cis configuration between the CO group and the chelate nitrogen atom.

Dinuclear complexes [M(SCN) (Ph2PO)2H ]2(M = Pd or Pt) have been prepared by metathetical reaction from the chloro-bridged dimers (see below).42 The X-ray structure (15) (see Table 1, p. 400) shows square-planar co-ordination around the Pd (N, S, 2 ) with an unusual symmetrically H-bonded anion [(Ph2PO)2H]. In view of the possibility of S- or N-bonding, molten thiocyanate is of some interest... 

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