for a complex ion to show coordination isomerism, is it necessary that the compound should contain two ions having same coordination number??pls answer

Total number of structural isomers of 
[Co(NH₃)₆][Cr(CN)₆]

Why is boiling point of Cesium higher than that of Rubidium

 

which isomer mentioned in (a) is water soluble and why?

A gaseous mixture contains CO2 and CO the mass of the mixture is 200g and they are presenting equal fraction of moles . Find the total no. of O-atoms present in the mixture.