Hydrated and Solvated Tin(II) Ions in Solution and the Solid State, and a Coordination Chemistry Overview of the d10s2 Metal Ions

The coordination chemistry of d10s2 metal ions is strongly affected by an (at least partially) occupied d10s2 metal ion–ligand atom antibonding orbital, which may cause a void in the coordination sphere due to repulsion between the electrons in the antibonding orbital on the metal ion and those on the ligands. The character of the formed d10s2 metal ion–ligand atom bond plays an important role in the electron density in the antibonding orbital and thereby also in the coordination chemistry. The hydrated tin(II) ion, [Sn(H2O)3]2+, and the trihydroxidostannate ion, [Sn(OH)3]−, have very different mean Sn−O bond lengths (2.21 and 2.08 Å, respectively) and O‐Sn‐O angles (ca. 78 and 90°, respectively) both in the solid state and in solution. On increasing the covalency of the tin(II)–ligand bonds, the repulsion decreases and higher coordination numbers are obtained, as seen in the dimethylsulfoxide‐ and N,N‐dimethylthioformamide‐solvated tin(II) ions, both of which are five‐coordinate with square‐pyramidal structures. Mind the gap! The coordination chemistry of tin(II) is strongly affected by the partially filled Sn(5s)–ligand(np) anti‐bonding orbital, which causes structural distortions including a large gap in the coordination sphere. The mean tin–ligand atom bond length varies widely with the type of ligand atom, even for the same coordination number (see figure). The actual coordination number and bond angles of tin(II) complexes seem to depend on tin–ligand bond character.