Square-pyramidal bonding

Formation of square pyramidal bonds by Sb and Bi valence group 2,10 If the valence group of M in a molecule or ion MX5 consists of six electron pairs the lone pair is expected to occupy the sixth octahedral position, giving a square pyramidal arrangement of bonds. Some complex halides of Sb are of interest in this connection (p. 708). 

A rather irregular pyramidal arrangement of three bonds is formed by Pb in Pb(N2S2)NH3, in which the system PbN2S2 is planar, Pb—S, 2-73, Pb—NH3, 2-24, and Pb—Nri g, 2-29 A. The square pyramidal bond arrangement is found in molecules of the diethyl thiocarbamate, (b), and the closely related ethyl-xanthate. Presumably an appreciable degree of ionic character is responsible for... 

The square pyramidal bond arrangement is found in a number of finite complexes. Apart from FeH(SiCl3)2(CO)CsH5 they are mostly monomers containing two bidentate ligands, (g), or dimers in which a fifth weaker bond is formed as shown at (h). The metal atom is situated about A above the centre of the square base of the pyramid. Dimerization of a molecule of type (g) gives a complex of type (i) in which the metal atom has distorted octahedral coordination. 

Synthesis of first PRJ type compound with a square pyramidal bond configuration around P... 

The deep violet color of pentaphenylbismuth and certain other pentaarylbismuth compounds has been the subject of considerable speculation. It has been shown by x-ray diffraction (173) that the bismuth atom in pentaphenylbismuth is square—pyramidal. WeU-formed crystals are dichromic, appearing violet when viewed in one plane but colorless in another plane. The nature of the chromophore has been suggested to be a charge-transfer transition by excitation of the four long equatorial bonds ... 

J. A. Le Bel, whose name is often also associated with this concept, did indeed independently suggest a 3-diitiensional modet for the 4-coordinate C atom, but vigorously opposed the tetrahedral stereochemistry of van t Hoff for many years and favoured an alternative square pyramidal arrangement of the bonds. 

The most important members of this class are the osmium nitrido, and the osmyl complexes. The reddish-purple K2[OsNCl5] mentioned above is the result of reducing the osmiamate. The anion has a distorted octahedral structure with a formal triple bond Os=N (161pm) and a pronounced /ram-influence (pp. 1163-4), i.e. the Os-Cl distance trans to Os-N is much longer than the Os-Cl distances cis to Os-N (261 and 236 pm respectively). The anion [OsNCls] also shows a rram-effect in that the Cl opposite the N is more labile than the others, leading, for instance, to the formation of [Os NCl4] , which has a square-pyramidal structure with the N occupying the apical position. 

All the OsNX4 complexes are distorted square pyramids (with N-Os-X angles of 103.7 to 104.5°) [188]. The stability of an osmium(VI) to iodine bond is unusual and is presumably owing to the extensive Os=N 7r-bonding reducing the positive charge on the metal and stabilizing it to reduction. 

The structures of several adducts can be rationalized on the basis [128] that in a 5-coordinate low-spin d8 tbp system, the acceptor ligands prefer to occupy an equatorial site (IrCl(CO)2(PPh3)2) whereas a 7r-donor prefers an axial site. In a square pyramidal situation, it is weakly bonded acceptors that prefer the apical position, e.g. (IrCl(S02)(C0)(PPh3)2. 

M(NO)Cl2(PPh3)2. Both these compounds have a square pyramidal structure with bent apical M-N-0 linkage and similar bond angles. There is, however, a difference of 70cm-1 in t/(N-0). 

A crystal-structure determination on [Ni(PhCH2CS2)2] showed evidence of a Ni-Ni bond (Ni—Ni distance, 256 pm) in a bridging, acetate-cage, binuclear complex (363). Each nickel atom is 5-coordinate and is in a tetragonally distorted, square-pyramid spectroscopic evidence for a Ni-Ni bond has been obtained (364). The polarized crystal spectra showed more bands than predicted for a mononuclear, diamagnetic, square-planar nickel(Il), and the spectra are indicative of substantial overlap of the d-orbitals between the two nickel atoms. The bis(dithiobenzation)nickeKII) complex was found to exhibit unusual spectrochemical behavior (365). 

Kawashima reported the structure of 1-phenyl-l//-phosphirene 102 which has a distorted square-pyramidal structure (Scheme 35) [82, 83], The observed elongation of the C=C bond has been interpreted in terms of the tt-o interaction in similar ways by Regitz [39] for cyclic phosphirene and by Clark [14] for phosphirenium ions 17. 

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