Sulfoxide complexes bonding

This review is intended as an account of the coordination chemistry of both dimethyl sulfoxide and the higher sulfoxides, with particular reference to the mode of bonding and the extent to which this affects the chemistry of transition-metal sulfoxide complexes. No attempt has been made to provide an exhaustive listing of all known sulfoxide complexes, many of which contain coordinated sulfoxide moieties only coincidentally to their other important functions. 

An oxygen-bonded sulfoxide complex will be termed anO-RjSO complex, and similarly, a sulfur-bonded complex will be termed an S-RjSO complex. Where the mode of coordination is not known or is uncertain, formulas of the type [M(R2SO)xC1 ] will be used. 

In order to understand the bonding in transition-metal sulfoxide complexes, it is necessary to summarize the physical data available in the literature and so determine what constraints are necessary in any bonding model. An understanding of the factors affecting the bonding in these complexes is essential if further developments are to be made in the chemistry of transition-metal sulfoxide complexes. 

In the chemistry of main-group element sulfoxide complexes, a relationship between Aj/(S=0) and the enthalpy of formation of the sulfoxide complex has been derived (159). The applicability of the equation has only been examined for O-l SO complexes, and the constraints on using Ai (S=0) as a measure of metal-ligand bond strength should be borne in mind during its application. 

This simple valence-bond rationale, involving a resonance hybrid of forms 1 and 2, appears to explain many of the physical data available for sulfoxide complexes. It appears that S-bonding does not involve such a major internal rearrangement of the molecule as one may initially expect and is almost certainly a result of the increased orbital diffuseness on passing from oxygen to sulfur. Thus with typical hard acids (see ref. 437), orbital overlap will be most favorable with the less diffuse donor orbital of oxygen. In the case of typical soft metals, this overlap is less favorable due to the orbital diffuseness of the soft acid, and so coordination via sulfur occurs, where the orbital diffuseness of the donor and acceptor are more evenly matched. 

Other workers have formulated mercury sulfoxide complexes as S-bonded, and this is discussed in Section VI in addition, NQR data (see Section I,H) suggest that these complexes are in fact dimeric in nature. 

Sulfoxide adducts of chromium, molybdenum, and tungsten carbonyls have been studied as catalysts for the polymerization of monomers such as vinyl chloride (248). Simple adducts of the type [M(CO)5(Me2SO)] may be prepared by carbonyl displacement from the corresponding hexacarbonyl. Photochemical reactions are frequently necessary to cause carbonyl displacement in this manner, many carbonyl complexes of higher sulfoxides have been prepared (255, 256). Infrared (257) and mass spectral studies (154) of these complexes have appeared, and infrared data suggest that S-bonding may occur in Cr(0) sulfoxide complexes, although definitive studies have not been reported. 

Undoubtedly, one of the major interests in platinum-metal sulfoxide chemistry is the synthesis and use of O-bonded sulfoxide complexes as reactive intermediates in synthetic and catalytic chemistry. The chemistry of such weakly bonded intermediates has recently been reviewed (142). 

CopperdI) dimethyl sulfoxide complexes [Cu(Me2SO)n]2+ n = 4 (309, 391) n = 5 (496) n = 9 (221)] have been synthesized and assigned as O-bonded by spectroscopic study. The Ph2SO complex [Cu(Ph2SO)4]2+ (228) has been prepared and isolated as its perchlorate salt. Preliminary crystallographic data (320) and ESR data (483) are available which suggest D4h symmetry for the metal-ion site. 

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