Square planar coordination structures

PtS (PdO structure) and PdS (similar) are prepared from M2+(aq.) and H2S or Li2S. They have square planar coordination of M2+ (Figure 3.9). 

However, it seems that these are best viewed as platinum(II) species too, so that two-electron metal-to-ligand transfer has been effected. The structures of Pt(PPh3)2Z (Z = r]2-02, t 2-C3H4, t]2-CS2) (Figure 3.16) all involve square planar coordination as expected for platinum(II) rather than the tetrahedral 4-coordination anticipated for platinum(O). 

The electron precise bonding, as sketched in Figure 38a, was based on the structural analogy between the Te72+ structure and those of the Te72 and Te65 anions that contain similar square planar coordinated Te atoms. However, it is unlikely to invoke the presence of a dianionic Te site within the... 

In the X-ray structures of both 3 and 4, the tetrahedral distortion is greater than that found in model A. Since there is an electronic preference for a square planar coordination, the pendant phenyl substituents in 3 and 4 likely result in further steric crowding and therefore in more distorted structures compared to model A. In the QM/MM model B an optimized 0 angle of 30° is found, close to the 34° angle of the X-ray structure. Since the phenyl and trimethyl phenyl groups are accounted for on a steric basis only in model B, the result supports the notion that the severely distorted coordination of the Pd center in 3 is due to a steric effect. 

A smaller C. N. than 6 is presumably exhibited by gold in the not yet fully known structure of AuFs, in which the gold ion is likely to have a square planar coordination of 4 fluoride ions 6). 

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