Which one of the following is an inner orbital complex as well as diamagnetic in behaviour ?

An inner orbital complex is one in which the central metal ion uses its inner (n-1)d orbitals for hybridization. Diamagnetic behavior means that the complex is not attracted to a magnetic field, indicating that it has no unpaired electrons. To answer this question, we need to consider the electronic configuration of each central metal ion and determine which one can form an inner orbital complex that is also diamagnetic. [Zn(NH3)6]2+ has a d10 configuration, which is diamagnetic, but zinc(II) is not typically considered an inner orbital complex because it usually uses its outer orbitals for bonding. [Cr(NH3)6]3+ has a d3 configuration, which can be diamagnetic if the electrons are paired, but chromium(III) can form inner orbital complexes. [Co(NH3)6]3+ has a d6 configuration, which can be diamagnetic if the electrons are paired, and cobalt(III) can form inner orbital complexes. [Ni(NH3)6]2+ has a d8 configuration, which is diamagnetic, but nickel(II) is not typically considered an inner orbital complex because it usually uses its outer orbitals for bonding. Considering the requirements for being both an inner orbital complex and diamagnetic, [Co(NH3)6]3+ fits the description best because it can form a low-spin complex that is diamagnetic and uses inner orbitals for bonding. Therefore, the correct answer is option (C) [Co (NH3)6]3+. Think of it this way: to be an inner orbital complex, the metal ion must use its (n-1)d orbitals for bonding, and to be diamagnetic, it must have no unpaired electrons.