Distorted square planar structures

With terpy, compounds of the type Ag(terpy)X have been prepared and cannot be tetrahedral for steric reasons, in that terpy cannot span three corners of a tetrahedron. It has been argued that more likely only two of the N atoms are coordinated and that a distorted linear structure is adapted.115 However, if this was the case the central N group would necessarily be within bonding distance. The other alternative was that they were three-coordinate. Addition reactions with neutral ligands such as H20, pyridine or phosphines would then yield distorted square planar structures. 

Allogonic behaviour has also been observed in complexes of the type [NiX2(amine)2]. For example, dark green diamagnetic [Nil2(2-methylpyridine)2] is considered to have a distorted square-planar structure in the solid state, but in dichloromethane it is tetrahedrally coordinated.255... 

Nickel(I) complexes of N4 macrocycles can be prepared by reduction of the corresponding Ni11 complexes with sodium amalgam. They possess more or less distorted square-planar structures.19 By contrast, the one-electron reduction of Ni porphyrin complexes may result in Ni1 porphyrins or Nin jr-anion radicals, depending on the reaction conditions.20 Complexes of this kind are useful models for the Ni sites in certain metalloenzymes (see below). 

Treatment of [Rh°(ri -TMPP)2] with terf-butylisonitrile yields a new paramagnetic species [Rh°(ri -TMPP)2(CNt-Bu)2] (16). X-ray diffraction revealed that [Rh°(ri -TMPP)2(CNt-Bu)2] " " adopts a distorted square-planar structure with mutually trans phosphine and isocyanide ligands. Somehow a cis-trans isomerization has taken place. The trans Rh—P bonds are slightly longer than the cis Rh P bonds in the starting material, as a result of a stronger trans influence of a phosphine donor compared to an ether. 

This is a 19-electron radical and can be contrasted with the radical N4, a 21-electron radical, which is thought to have a distorted square-planar structure, with all the nitrogen atoms equivalent (81). Another center, with four equivalent sulfur atoms, has recently been described, although details are not given (37). A square-planar structure again seems possible and it would be of interest to discover the nature of the orbital of the unpaired electron. Certainly the factors which determine the most stable geometry of these tetra-atomic radicals are a matter of some interest. 

The associative pathway of ligand exchange starts from the addition of a nucleophile to the positively charged iodine atom of a X -iodane with the initial formation of a trans hypervalent 12-1-4 square-planar species. This intermediate species isomerizes to the cis 12-1-4 square-planar intermediate and eliminates the ligand L to afford the final product (Scheme 1.4). Such a mechanism has been validated by the isolation and X-ray structural identification of several stable 12-1-4 species. For example, the interaction of ICI3 with chloride anion affords tetrachloroiodate anion, ICl4, which has a distorted square-planar structure as established by X-ray analysis of the trichlorosulfonium salt, CI3S+ ICU [209]. 

Reaction of bis(disilanyl)dithiane 6 with an equimolar amount of Pd(CN-t-Bu)2 in benzene was examined at room temperature. The reaction was completed within 5 min, affording l,l,2,2-tetramethyl-l,2-diphenyldisilane and a four-membered cyclic bis-(organosilyl) complex 7 in 85% yield after isolation by crystallization with pentane (Scheme 5). A crystal structure of 7 exhibited a distorted square planar structure, in which the two silicon atoms in the ring are separated only by 2.61 A. Presumably, the formation of 7 may have arisen from the simultaneous activation of the two Si—Si bonds on palladium, which led to intramolecular metathesis (disproportionation). 

With P-donors a variety of different structures are formed. Two complexes (359a,b) with a five-coordinate square pyramidal geometry and one S-donor in the apical position are reported.934,933 In a related complex (360) the nickel center reveals a distorted square planar geometry, because one xanthate ligand switched to a monodentate coordination mode. 6 By reaction of [NiL2] complexes with Ph2P(CH2)2P(Ph)(CH2)2PPh2, compound (362) is formed, which consists of a five-coordinate cation and a hexacoordinate anion. 7... 

The ligand 2,5-bis(2-pyridyl)pyrazine was used in the synthesis of a range of zinc compounds to contrast with copper coordination polymers prepared with this ligand.169 Three zinc compounds were structurally characterized, [ZnCl2L(DMF)], [Zn3(OAc)6L2], and [Zn3Cl6L3] respectively a distorted square planar monomer, a linear trimer, and a triangular arrangement of zinc centers. 

Among the numerous nickel dithiophosphates, the chiral complexes deserve to be cited.41 Unusual trinuclear Pd3(p3-E)2[S2P(OR)2]2(PPh3)2 with E = S, Se, R = Et, Pr, Pr, Bu, have been prepared and Pd3(p3-S)2[S2P(OEt)2]2(PPh3)2 revealed a structure containing three distorted square-planar Pd atoms and sharing two p3-S ligands.42... 

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