Trend 6.7 of the Oxygen Group (Group VIA) Trends says that the increasing size of the atoms leads to compounds with progressively larger maximum coordination numbers (coordinative unsaturation). Oxygen usually has a coordination number of 2 or 3 with a few examples of 4 coordination, sulfur exhibits a maximum coordination number of 6, and higher coordination numbers of 8 have been observed for Te.
These maximum coordination numbers have an impact on the case of hydrolysis of the halides, e.g. the rate of hydrolysis is: TeF6 > SeF6 > SF6 Also, the octahedral anionic complexes [MX6]2- (X = halide) are more commonly observed for Se, Te, and Po.
The coordination number of an atom is the number of areas of electron density between other atoms that a molecule can form. A higher coordination number implies that an element has more possible areas of electron density around it and a smaller coordination number implies that an element has fewer possible areas of electron density around it. One of the most important contributors to coordination number has to do with the size of the species in question.
In Group VI, the coordination number of atoms increases as you move down the column. Oxygen has a maximum coordination number of four, but is most commonly found with a coordination number of two or three. Sulfur and Selenium species commonly have a coordination number of six. Tellurium and Polonium have maximum coordination numbers of eight.
As you move down the column, you are adding additional shells of electron density, which creates a larger atom. Larger atoms have a greater amount of surface area with which other atoms can interact. This increase in surface area is the primary reason for the increase in coordination number.
Oxygen is unique compared with the other atoms in the same column because it is much smaller than the other atoms. This is due to the few electron blocks and due to the extremely high electronegativity of oxygen. The charge to size ratio of oxygen is very high, which makes the atom very small, thus making the coordination number the smallest as well.
Sulfur has some shielding caused by the 3s block electrons. These electrons decrease Sulfur’s ability to form pπ-pπ bonds, which enhances the atom’s necessity for a higher coordination number. Selenium is similar because it has additional shielding from the 3d block of electrons, and actually has some stable isotropes that contain a coordination number of eight.
Tellurium and Polonium
Tellurium and Polonium have a maximum coordination number of eight due to their increased size and additional size from shielding in the 4d block and 4f blocks respectively. These additional coordination numbers allow for additional bonds, but there is also an increase in the rate of hydrolysis.
Hydrolysis of halides
The larger the atom, the higher the rate of hydrolysis of halides. This is due to the electrons being more spread out throughout the surface of the central atom. The atom’s charge is so dispersed that the bonds become weaker than they would be if there were a smaller coordination number or a smaller size. The decrease in charge to size ratio as you move down the column also accounts for the increase in the rate and ability to perform a hydrolysis of the halides.