Stannylenes complexes

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. 

It is interesting to note that no examples are known for a retro-reaction of this dimerization. Such a reaction has been observed, however, for germylene complexes and for stannylene complexes, in some cases an equilibrium between uncomplexed and base-stabilized compounds has been found. 

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. 

The structures of these stannylene complexes closely resemble those of carbene complexes. In Fig. 12 the crystal structure of the stannylene complex 4 is displayed the tin atom, the two carbon and the chromium atoms are equi-planar 30). 

A remarkably stable, deep red Ni° stannylene complex, [Ni(1068)4l, has been prepared by the reaction of [Ni(l,5-cyclooctadiene)2] with (1068) in toluene at —78 °C. 70 In spite of the bulkiness of (1068) and the known tendency of analogous Ni° phosphine complexes to dissociate in solution, [Ni(1068)4] remains intact in solution and, moreover, melts at 178-180 °C without decomposition. X-ray crystallography shows tetrahedral geometry about the nickel atom, with Ni—Sn bond lengths of 2.3898(2)-2.399(2) A. 

Bis(amido)-germylene and -stannylene react with [Pt(COD)2] to give the platinum(O) complexes [Pt(M NTMS 2)3], M = Ge or Sn.78 The stannylene complex has a trigonal-planar structure with Pt—Sn bond lengths of 2.47-2.50 A.79... 

Sergey E. Boganov, Mikhail P. Egorov, Valery I. Faustov and Oleg M. Nefedov TABLE 6. Absorption maxima of germylene and stannylene complexes with n-donor agents... 

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. 

The reaction of Sn(r 1-Cp)2 with diiron enneacarbonyl produces bridging stannylene complexes (equation 128)298 304 305. 

Treatment of Cp(CO)(SnMe3)Fe PN(Me)CH2CH2NMe(OR) (R = Me, Et) with Me3Si0S02CF3 resulted in P—OR bond cleavage and migration of a methyl group from tin to phosphorus, to yield an unusual stannylene complex 100312. 

Cyclic diazastannylenes, Sn(NBu-f)2SiMe2, afford a series of interesting stannylene complexes. They insert into the Fe—Me bond of Cp(CO)2FeMe, to form the corresponding stannylene complex 101101. 

The reaction of (CO)sMnSnMe3 with BF3 gives the stannylene complex [Me2Sn Mn(CO)5]BF4 the same compound can be obtained from (CO)5MnSnMe2Cl with AgBF4 (equation 132)302. 

A limited number of thermal redistribution reactions are reported, involving redistribution at both the tin and metal centers of the Sn-M complex (equations 136 and 137)329-331. The product in equation 137 contains a cyclic Os3Sn3 skeleton. A similar multimetal product 109 was obtained by the pyrolysis of the stannylene complex [/71-Cp2SnFe(CO)4]2332. 

Photolysis of Me2Sn[Mn(CO)s]2 proceeds with loss of carbon monoxide and four-membered ring closure to form 111, and similar bridging stannylene complexes 112 and 113 are formed by the photochemical treatment of related Fe and Co complexes334. 

The hydroxo hydrido stannylene complex Co(/r-OH)(H)Co Sn[CH(SiMe3)2]2 2 and its deuteriated analog were obtained by treating the ethylene complex... 

Eq. (25)].57 This type of reaction is more common to stannylene complexes.8 The inverse reaction, that is, formation of base-stabilized mononuclear silylene complexes from doubly silylene-bridged dinuclear complexes, is described in Section V,B. 

In the same year Kuhn and co-workers reported a series of silicon- and tin-NHC complexes (Scheme 15).33 The pentacordinated silicon and tin structures 24 and 28 were determined by single crystal X-ray crystallography, and possess metal-NHC bond lengths of 1.911(7) A and 2.179(3) A respectively. The monomeric stannylene complex 29 was also... 

The X-ray crystallographic structure of the base stabilized terminal stannylene complex f-Bu2Sn(py)Cr(CO)5 is shown in Fig. 12 (95). The slightly longer Sn—Cr bond length in this complex, and the slight eleva-... 

In addition to serving as ligands, organostannylenes can also insert into metal-ligand bonds (cf. Fig. 10). In the reaction shown in Eq. (50) (78), two moles of the divalent tin compound react, one forming a terminal stannylene complex, and the other inserting into the Pt—Cl bond. 

Stannocene reacts with Fe2(CO)9 to give the bridging stannylene complex [SnCp2Fe(CO)4]2, the structure of which was shown in Fig. 13. Bis(bistrimethylsilylmethyl)tin(II) does not react analogously, giving instead the complex shown in Eq. (53). 

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). 

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

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. 

OR, -SEt, and -NR2 migration from a tertiary phosphine to phosphenium P (from 32 to 31 in Scheme l).33 Another one is 1,3-alkyl migration from a stannyl ligand to phosphenium P (from 36 to 39 in Scheme 2).23,24 34,36 48 In this reaction, a phosphenium complex is converted into a stannylene complex. This reaction corresponds to migration of a double bond from M=P to M=Sn. The last one is 1,3-alkoxy migration from a phosphite... 

The first ionization potential of R2M is rather low. Its values for [(Me3Si)2CH]2M (M = Ge, Sn, Pb) are 7.75, 7.42 and 7.25 eV, respectively. In contrast to the numerous known structures of germylene and especially stannylene complexes, the structures of only a few silylene complexes are known. 

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