Stannylenes complexes with transition metals—

As heavier analogs of carbenes141) stannylenes can be used as ligands in transition-metal chemistry. The stability of carbene complexes is often explained by a synergetic c,7t-effect cr-donation from the lone electron pair of the carbon atom to the metal is compensated by a a-backdonation from filled orbitals of the metal to the empty p-orbital of the carbon atom. This concept cannot be transferred to stannylene complexes. Stannylenes are poor p-a-acceptors no base-stabilized stannylene (SnX2 B, B = electron donor) has ever been found to lose its base when coordinated with a transition metal (M - SnXj B). Up to now, stannylene complexes of transition metals were only synthesized starting from stable monomoleeular stannylenes. Divalent tin compounds are nevertheless efficient cr-donors as may be deduced from the displacement reactions (17)-(20) which open convenient routes to stannylene complexes. 

Base-stabilized stannylenes have been known to form complexes with transition metals before stable stannylenes were detected. They are synthesized by a reaction similar to process (17) or by reduction of Sn(IV) compounds according to Eq (23) 146) ... 

The NMR spectra of the two-coordinate stannylenes in solution show values of Sn ranging from about 1150 (e.g., in ArSnl) to 3750 (in (Ar3Sn)Sn ), with a large anisotropy. The stannylenes behave as Lewis acids, for example, in the three- or four-coordinate complexes (e.g., 78, 79, and 80), which are formed when the molecule carries an intramolecular ligand, and as Lewis bases, particularly in complexing to transition metals (e.g., 81, 82, and 83). The dimerization of stannylenes to give distannenes can be regarded as a result of this amphoteric character (Equation (179)). 

The stannylenes and their transition metal complexes have been dealt with in a series of recent reviews5,295. We concentrate upon the most recent developments which chiefly involve direct reactions of preformed stannylenes with various metal complexes (equation 125). The addition of SnCp2 or SnCp2 to THF solutions of M(CO)5THF (M = Cr, Mo, W) yields stannylene complexes R2SnM(CO)5 (R = Cp, M = Cr, Mo R = Cp, M = Cr, W)296-298. 

Divalent organotin and organolead compounds can be divided into two classes, viz, those in which the organic groups are organic groups are 7r-bonded to the metal. We will deal with these two classes separately beginning with the 7r-bonded species. A third section will cover transition metal stannylene and plumbylene complexes from both classes. 

As mentioned above, cationic phosphenium is isoelectronic with carbene, silylene, germylene, stannylene, and plumbylene, because the central element has lone pair electrons and a vacant p orbital as well as two substituents. Therefore, their transition metal complexes have attracted considerable attention. It might be of special interest to compare cationic phosphenium complexes with silylene complexes because phosphorus and silicon are both situated in the third row of the periodic table. 

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