Complex ions

Q1.
4OH- + [Fe(H2O)6]2+ --> [Fe(OH)4(H2O)2]2- + 4H2O
lone pair of OH- is more concentrated and coordinates Fe2+ with more energetically stability with high charge density central ion Fe2+. therefore, the stability contant when first few OH- ligands are attached to Fe2+.
when more OH- replace H2O, the overall charge of the complex attains a negative charge which electrostatically repel further replacement of OH-.

Q2.
[Cr(H2O)6]3+ is hexa-coordinated and is geometrically octahedral.
this can be deduced that Cr3+ is d2sp3 hybridised.
let x be arrow pointing down and y pointing up for the sake of representation of electrons of Cr3+
and X and Y be those belong to water ligands pointing up and down respectively.
underline, _, means half filled.

before hybridisation: 3d: |x_|x_|x_| | | 4s:| | 4p:| | | |
hybridisation requires low lying vacant 4p orbitals
after hybridisation: 3d: |x_|x_|x_| d2sp3: |X Y|X Y|X Y|X Y|X Y|X Y|