Organotin compounds four-coordination

Tin, having valence of +2 and +4, forms staimous (tin(II)) compounds and stannic (tin(IV)) compounds. Tin compounds include inorganic tin(II) and tin(IV) compounds complex stannites, MSnX., and staimates, M2SnX, and coordination complexes, organic tin salts where the tin is not bonded through carbon, and organotin compounds, which contain one-to-four carbon atoms bonded direcdy to tin. 

Chemical interactions with solvents Various workers have observed (1, 4, 7) that five- and six-coordinate organotin compounds show U9Sn signals which occur at much lower frequencies than those of the four-coordinate derivatives. For this reason, the choice of solvent in preparing samples for NMR measurements may affect the value of the observed u9Sn chemical shift. 

Consequently, increasing substitution of R by X in four-coordinate organotin compounds, R SnX4 , produces an initial decrease in shielding followed by a progressive increase, to give a characteristic U-... 

The first examples of heteroleptic divalent organotin compounds authenticated by X-ray crystal structure determinations are [2-pyridylbis(tri-methylsilyl)methyl]tin chloride 12 and [2-pyridylbis(trimethylsilyl)methyl] bis(trimethylsilyl)amidotin 13, for which the structures are schematically shown in Fig. 8 (73). The distorted trigonal pyramidal coordination geometry of the tin center in these compounds may be a result of the formation of a four-membered chelate ring. 

The ability of organotin compounds to self-assemble, with an increase of the coordination number of tin from four to five and six, has been recognized quite early, and the resulting compounds were termed autocomplexes [345]. 

Divalent organotin compounds (organostannylenes) represented by R2Sn and the compounds of which R is alkyl or aryl, are liable. But the organic groups shown in eqs. (10.18)-( 10.20) are stably obtained [37—40]. The compound shown in eq. (10.18) has a three-coordination structure of dimer ((Mc3Si)2CH)2Sn-Sn(CH(SiMe3)2)2). On the other hand, the compounds shown in eq. (10.20) is four coordination with the coordination of pyridine. 

Halogen or nitrogen atoms of cisplatin are in the cis position. But, organotin compounds are not formed in planar four coordination as the cisplatin. However, the octahedral structure of six-coordination is able to form partly cis structure on halogen or nitrogen atoms similar to cisplatin. 

The most substantial study of this sort, analysing structures of 186 four-, five- and six-coordinated tin compounds, was published by Britton and Dunitz (1981) under the memorable title Pathways for SN2 and SN3 Substitution at Sn(IV) . Organotin halides readily increase their coordination number to 5 or 6 (Davies and Smith, 1982), and a wide range of structures are stable in the solid state. 

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