Five-coordinate species pyramidal... trigonal bipyramidal

Five-coordinate species represent a rare class of coordination compounds. This rather unusual coordination number is a delicate balance between electronic factors which favor disproportion into four- and sbc-coordinate species and directional covalent bonds which help to stabilize them. The two limiting geometries (trigonal bipyramidal and square pyramidal) are therefore similar in energy and easily interconverted. As a result, five-coordinate species can exist as fluxional molecules, where their ligands can exchange places rapidly on the NMR time scale. 

Six-coordinate complexes are expected to be distorted from pure octahedral symmetry by the Jahn-Teller eEFect and this distortion is generally observed (Chapter Ml. A number of five-coordinate complexes are known, both square pyramidal and trigonal bipyramidal. Four-coordination is exemplified by square planar and tetrahedral species as well as intermediate configurations. 

Addition of triphenylphosphine or t-butylisocyanide to 101 affords the analogous five-coordinate complexes 102b and 102c. These species represent rare examples of stable five-coordinate ds-dialkyl complexes of Ni(II), although stable trigonal-bipyramidal trans-dialkyls of nickel are well known (176-178), and they provide a structural model for a putative square-pyramidal intermediate in reductive elimination reactions of ds-dialkyl... 

A series of air-sensitive trigonal bipyramidal and trigonal pyramidal Ni complexes, [Ni(PR3)4]+, [NiX(PR3)3], [NiX(AsR3)3] (X = halide), and [Ni(tripod)]+ and [NiX(tri-pod)] (tripod = face-capping tripodal P, As ligand), have been reported. The methods of synthesis of these species are varied, but essentially are based on the reduction of Ni or oxidation of Ni precursors. A range of four- and five-coordinate Ni complexes have been stmcturally characterized and some of these are illustrated in (42)-(46). 

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