Stannylenes Transition metal stannylene

Considering the interesting bissilylene complexes, the question arises how many silylenes can be coordinated to a transition metal. In the series of stannylene complexes, a maximum coordination number of at least 3 could be established. 

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)). 

Novel, monomeric heteroleptic derivatives of divalent Ge 40 and divalent Sn 41 have been prepared and characterized by single crystal X-ray diffraction <19990M389>. Heteroatoms in these germylene and stannylene species are not formally part of a four-membered ring however, crystal structures point to the azametallacyclobutane structures formed due to the N —> M coordination. The heteroleptic nature of 40 and 41 gives rise to an interesting reactivity in their coupling with transition metal clusters. 

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. 

The bridging stannylene complex [Cp2SnFe(CO)4]2 undergoes the thermal rearrangement shown in Eq. (56) in which the tr-bonded cyclopenta-dienyl group is transferred from tin to the transition metal. 

Tin-119 Mossbauer data have been reported for a number of stannylene complexes and are collected in Table VI. The IS (isomer shift) of the stan-nocene complexes is much lower than that of stannocene, while the QS (quadrupole splitting) is much higher. This has been interpreted as indicating synergistic cr- plus 7r-bonding of the stannylene to the transition metal (52). 

There is little change in the IS values of tin on complexation of Sn[CH(SiMe3)2]2 to transition metals, but it must be remembered that the free ligand is a dimer (88). There is a dramatic increase in the QS values upon coordination of Sn[CH(SiMe3 )2]2 to a transition metal, possibly reflecting back donation from the transition metal to empty orbitals on the stannylene. 

Tin-119 Mossbauer Data for Transition Metal Stannylene Complexes... 

A major problem in postulating silylenoid metal complexes as intermediates in the redistribution reactions is simply that good model compounds are lacking, and the decomposition mechanisms of silyl transition metal complexes have not been systematically investigated. While there is evidence for transient R2Si species produced by thermal or photochemical means (80-83), there are no known monomeric silylene metal complexes. Several monomeric stannylene and germylene complexes are... 

In contrast to the alkyl, silyl, and germyl complexes, when E is Sn, not SnR3 but only one alkyl group on the Sn migrates to the phosphenium P to give a stannylene complex (39).23,34 Since the transition metal-phosphorus bond has considerable double-bond character (see below), the conversion from 36 to 39 corresponds to a double-bond migration from Fe=P to Fe=Sn. 

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. 

The most characteristic reaction of stannylenes as Lewis bases is their coordination to a variety of transition metals 48-51 Some examples are given in Scheme 21-6. 

---