Assign oxidation numbers to each element in H2PtCl6.

The stereocenter in the molecule above is the carbon atom with the two methyl groups and the bromine atom attached to it. To assign R or S configuration to this stereocenter, we need to assign priorities to the four groups attached to the stereocenter based on the atomic number of the atoms attached to it. In this case, the bromine atom has the highest atomic number, followed by the carbon atom attached to the two methyl groups, and then the two methyl groups themselves. By convention, we assign priority based on the direction of the lowest priority group, which in this case is one of the methyl groups. If we rotate the molecule such that the lowest priority group is pointing away from us, we can then determine the configuration of the remaining three groups. If the order of priority of the remaining three groups is clockwise, then it is assigned an R configuration, and if it is counterclockwise, it is assigned an S configuration. In this case, if we rotate the molecule such that one of the methyl groups is pointing away from us, the remaining three groups are arranged in a counterclockwise manner. This means that the stereocenter has an S configuration.