Arrange these complexes in order of octahedal splitting energy, 4,. 
Largest a° 
Smallest A. 
[Cr(OH)613 
[Co(NH3)1,]3+ 
[Cr(NO2)6]3 
[Ru(NO2)6]3- 
 

1st complex is Cr3+ weak ligand hence delta o is small

2nd complex is Co3+ in reasonably strong ligand hence delta o is higher

3nd complex is Cr3+ with stronger ligand hence delta o is much higher

final is Ru 3+ with stronger lignad where Ru has 4d6 orbital delta o is much higher ( as 5d>4d>3d which causes delta o higher)

hence order is

smallest [Cr(OH6)]3- < [Co(NH3)6]3+ < [Cr(NO2)6]3- < [Ru(NO2)6 ] 3-