Square pyramid stabilization

SF4 is unusual in apparently acting both as an electron-pair acceptor and an electron-pair donor (amphoteric Lewis acid-base). Thus pyridine forms a stable 1 1 adduct C5H5NSF4 which presumably has a pseudooctahedral (square-pyramidal) geometry. Likewise CsF (at 125°) and Me4NF (at —20°) form CsSFs and [NMe4]+[SFs] (Fig. 15.21a). By contrast, SF4 behaves as a donor to form 1 1 adducts with many Lewis acids the stability decreases in the sequence SbFs > AsFs > IrFs > BF3 > PF5 > ASF3. In view of the discussion on... 

Square pyramid A pyramid that has a square as its base, 176,181 Stability, belt of, 30... 

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. 

Vahrenkamp and co-workers formed a number of chelate-stabilized ketone complexes with pyridylphenyl ketone. Octahedral complexes formed with a 2 1 ratio of ligand to zinc were formed with trifluoromethanesulfonate, chloride, and bromide anions. 1 1 reactions resulted in the formation of square pyramidal [ZnL2X]+ with X = Br, I, and a trigonal-bipyramidal dimer [L2Zn(NCS)2]2 with thiocyanate bridges.350... 

The hypervalent chalcogen chemistry has been developed to higher coordinated species with various ligands,7 10 where TBP changes to square pyramidal (SP) or octahedral (Oh), etc. Additional atomic orbitals of E, such as an 5-orbital, may operate to stabilize the structures.10b The concept is also extended over linear a-bonds constructed by m ( > 4) atoms with n electrons (extended hypervalent bonds mc-ne (in > 4)).11 14 The approximate molecular orbital model for mc-ne (m > 4) is also exhibited in Scheme la, exemplified by 4c-6e. 

BMOV, was reported in 1972 159) and in 1987 160). Its electrochemical preparation was described in 1978 92a), and EPR monitoring of its redox behavior, in chloroform, in 1987 160). However this now-important compound seems not to have been properly characterized until 1992 161). Since then complexes of several 3-hydroxy-4-pyridinones 162—164), and of l-hydroxy-2-pyridinone 165), have been synthesized and characterized, especially by EPR 164). VO(malt)2 exists as a cis trans equilibrium mixture in aqueous solution, and generally crystallizes as a mixture of the two isomers. However the crystal structure of the trans structure was eventually solved, confirming the expected square-pyramidal stereochemistry 166). The relative stabilities of the cis and trans forms of V 0L2 complexes depend on the nature of the bidentate ligand L , with the cis configuration favored by VO(malt)2 and VO(koj)2 167), but the trans by 3-hydroxy-4-pyridinonate ligands 164). 

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