Planar four-coordinate

The distance between the diagonal nitrogen atoms is approximately 4 A [ 12]. The distortion induced by the incorporated metal is negUgible and planar four-coordinated metallocomplexes are produced. Therefore, the metal-nitrogen dis-... 

Intermediate spin occurs mainly for square- and rhombic-pyramidal five-coordinate iron complexes and also for planar four-coordinate and for asymmetric six-coordinate... 

The second category is the transition metal ions, all of which in Fig. 1 are six-coordinate with the exception of Pt2+ and Pd2+, which are square-planar four-coordinate (6-9). Their labilities are strongly influenced by the electronic occupancy of their d orbitals. This is illustrated by the divalent first-row transition metal ions, which should exhibit similar labilities to Zn2+ on the basis of their rM instead, however, their labilities encompass seven orders of magnitude. On a similar basis, the trivalent first-row transition metal ions might be expected to be of similar lability to Ga3+, but instead they exhibit a lability variation of 11 orders of magnitude, with Cr3 being at the... 

The divalent Co(salen) complex (69a) is one of the most versatile and well-studied Co coordination compounds. It has a long and well-documented history and we shall not restate this here. Recent applications of (69a) as both a synthetic oxygen carrier and as a catalyst for organic transformations are described in Sections 6.1.3.1.2 and 6.1.4.1 respectively. Isotropic shifts in the HNMR spectrum of low-spin Co(salphn) (69b) were investigated in deuterated chloroform, DMF, DMSO, and pyridine.319 Solvent-dependent isotropic shifts indicate that the single unpaired electron, delocalized over the tetradentate 7r-electron system in CHCI3, is an intrinsic property of the planar four-coordinate complex. The high-spin/low-spin equilibrium of the... 

For intermolecular hydrocarbon activation, we are not aware of any example where compelling evidence exists for C-H activation directly by square-planar four-coordinate Pt(II), without preceding (or concomitant) ligand loss. In one example, such a direct reaction may take place but the alternative explanation involving ligand loss is also consistent with the data. The compound (dmpe)PtMe(02CCF3) (dmpe = bis(dimethylphosphino)ethane) activates benzene C-H bonds at elevated temperature (125 °C) (23). This reactivity contrasts with that of (dmpe)PtMeCl which is inactive at the same temperature or even at... 

Platinum(II) complexes containing the tridentate ligands diethylenetriamine (dien) and 2,2 6, 2"-terpyridine (terpy) have provided useful substrates for mechanistic studies in substitution reactions at planar four-coordinated complexes. Moreover, there has been considerable interest in terpy complexes... 

Nickel(II) complexes display a variety of equilibria which involve spin state changes. Planar four-coordinate complexes are invariably diamagnetic. These can undergo an intramolecular isomerization to paramagnetic tetrahedral four-coordinate species. Alternatively, the planar complexes can coordinate additional ligands to form five- and six-coordinate paramagnetic complexes. The additional ligand molecules can be Lewis bases in solution, or solvent molecules, or, in par-... 

A key feature of the cationic mechanism is that removal (or dissociation) of an anionic ligand from the palladium coordination sphere allows alkene complexation to occur while maintaining coordination of both phosphines of a bisphosphine ligand. That both phosphines can be accommodated in a square-planar four-coordinate intermediate during the insertion step has provided a simple rationalization for the higher enantioselectivities often observed for the cationic pathway. Concrete information on the enantioselective step of asymmetric Heck reactions proceeding by the cationic pathway has not been reported to date. It is likely to be either coordination of the alkene to generate 20.S or the insertion step (20.5 —> 20.6, Scheme 8G.20). 

The neutral pathway differs from the cationic pathway in the absence of a vacant coordination site in the square-planar four-coordinate palladium(II) intermediate prior to alkene coordination. The key question is then how does alkene coordination take place. Early studies pointed out that Heck reactions of aryl or vinyl halides promoted by (bisphosphine)palladium complexes could be sluggish, and this sluggishness was attributed to a reluctance of one of the phosphines of the... 

Nickel(II) is a 3d8 ion and has two unpaired electrons (5 = 1) when it is six coordinated or four coordinated pseudotetrahedrally. In the latter configuration, sharp proton NMR signals are obtained. When it is planar four coordinated nickel(II) is always low spin diamagnetic (5 = 0). Five-coordinated nickel(II) complexes can either be high (5 = 1) or low spin (5 = 0) depending on the nature of the donor atoms. 

Distorted Square Planar Four-Coordinate The first example of helical chirality for a square planar complex with achiral ligands is cis-bis(2,6-diphenyl-... 

Square-planar, four-coordinate tellurium(II) complexes, whose structural chemistry has recently been summarized by Foss 89>, offer illustrations of this highly approximate, constant-volume rule. Fig. 27 is a drawing, approximately to scale, of a schematic, electron-domain representation of a section through the 1 1 complex of benzenetellurenyl chloride with thiourea 90>. Shown are the electron domains of the tellurium kernel (largest, nearly centrally located solid circle) the ligands kernels (two chlorine kernels, a sulfur kernel of thiourea, and a carbon kernel of benzene) and the domains of the tellurium atom s shared valence-shell electrons (open circles) and, most schematically of all, the tellurium atom s unshared valence-shell electrons (shaded region). 

Figure 11.1. Schematic representation of typical silver (central atom) coordination environments with different coordination numbers and geometries (a) linear two-coordinate (b) triangular three-coordinate (c) T-shaped three-coordinate (d) square planar four-coordinate (e) tetrahedral four-coordinate (f) trigonal bipyramidal five-coordinate (g) tetragonal pyramidal five-coordinate (h) octahedral six-coordinate (i) trigonal prism six-coordinate (j) seven-coordinate (k) tetragonal prism eight-coordinate.

The tellurium atom in the bis[maleonitriledithiolato]tellurate(2-) anion has a trapezoid planar four-coordination with no intermolecular contacts1. 

The principal oxidation states are Pt(II) and Pt(IV). Pt(II) is the prototypical d ion and forms square-planar, four-coordinate complexes, which are somewhat sluggish in their ligand exchange reactions. Pt(IV) is a d metal ion and forms octahedral 6-coordinate complexes, which exhibit the typical kinetic inertness associated with the d configuration. Pt(IV) is an oxidizing agent and is reduced to Pt(II) under biological... 

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