Ligand structures sulfur dioxide

Reduction of both nickel porphyrins and thiaporphyrins to Ni1 species has been studied by EPR and 2H NMR spectroscopy.179, 2 58 The Ni1 complex of 5,10,15,20-tetraphenyl-21-thiaporphyrin has been isolated and characterized. Reaction of this complex with sulfur dioxide produced a paramagnetic five-coordinated Ni1 S02 adduct, while reaction with nitrogenous base ligands (amines, pyridines, imidazoles) yielded five- and six-coordinate complexes. In addition, the crystal structure of Ni1 diphenyldi-p-tolyl-21-thiaporphyrin has been determined. The coordination geometry about the nickel center is essentially square planar with extremely short Ni—N and Ni—S bonds (Ni—N = 2.015(2) A, 2.014(12) A, and 1.910(14) A and Ni—S = 2.143(6) A).2359... 

The amphoteric nature of the nitrosyl ligand has been extensively discussed by several authors in terms of molecular orbital models and in particular the utility of the extended Hiickel approach for understanding the bonding has been demonstrated by Hoffmann, et al. This latter work has been extended to sulfur dioxide complexes by Ryan and Eller The interested reader is referred to the recent review by Mingos for a summary Although detailed molecular orbital treatment will no doubt be necessary for a complete understanding of SO2 bonding, we find that the simple acid/base concept serves to correlate the presently known structural information. This point of view will be emphasized in the present article and extended to include >7 -802 as a r acid 1... 

An X-ray structure determination Cmcm i i= 0.061 of pale yellow crystals of compound B obtained from the sulfur dioxide reaction, shows a neutral complex of symmetry mm2 to be present in the solid. The anionic part of the molecule consists of two anellated l,2-dihydro-5-oxo-l,2,4-triphosphol-3-olate rings which share the central P-P group (P-P 215.3 P-C 189.1 P-C 178.4 C-O 123.9pm C-P-P 98.4° C-P-C 91.2° C P C 98.7°). The lithium cations are coordinated square pyramidally (Li-0 193.5 to 209.1 pm), each binding a 1,2-dimethoxyethane and a tetrahydrofiiran ligand in addition [30]. 

Sulfur dioxide is known to behave as an S—O bidentate to metal ions in low oxidation states, resulting in the formation of three-membered chelate rings. The structures of three such complexes of molybdenum have been described by Kubas and Ryan,including an unusual dinuclear complex in which the SO2 ligands bridge two Mo atoms. A closely related complex of Ru also has been characterized. This bonding form was discussed in relation to these structimes. 

Insertion of sulfur dioxide (SO2) into the metal-carbon bond of transition metal alkyl and aryl complexes has also been studied extensively. SO2 shows several binding modes to transition metals as shown in Scheme 7.15 because it is amphoteric, behaving as a Lewis acid and a Lewis base. The Lewis base character of SO2 provides the structural types r/ -planar (3) or (S,0) (4) where SO2 donates a pair of electrons to the metal accompanied by rr back-bonding from filled d orbitals of the metal atom. The Lewis acid behavior of SO2 as a ligand affords an 17 -pyramidal bonding mode (5) where SO2 accepts a pair of electrons from the metal. As ligands tike olefins or carbon dioxide generally tend to prefer... 

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