Question

a) Valence Bond Theory (VBT) can explain the magnetic behaviour and shape of complexes. Using VBT explain the diamagnetism and square planar shape of [NKCN)₄]^2-
b) i) Suggest the shape of the following complexes Ni(CO)₄ and [CoF₆]^3-.
ii) The central ion Co³⁺ with coordination number 6 is bonded to the ligands NH₃ and Br to form a dipositive complexion. Write the formula or IUPAC name of the complexion.

Answer 1

a) In the complex ion [Ni(CN)₄]² the central metal atom Ni is in +2 oxidation state and has the electronic configuration 3d⁸. Since CN – is a strong field ligand two unpaired electrons are forced to pair up against Hund’s rule and makes available a vacant 3d, one 4s and two 4p orbitals, which hybridise to give four dsp2 hybrid orbitals. Each of the hybrid orbitals receives a pair of electrons from a cyanide ion to form [Ni(CN)₄]^2-. Since there is no unpaired electron, the complexion is diamagnetic.

Since the hybridisation involved is dsp² this complex has a square planar geometry.

b) i) In [Ni(CO)₄] the central atom Ni undergoes sp³ hybridisation. Hence, it has a tetrahedral geometry. In [CoF₆]³ the central metal ion Co³⁺ is in the sp³d² hybridised state. Hence, it has an octahedral geometry,
ii) [Co(NH₃)₅Br]²⁺

OR
Pentaamminebromidocobalt(lll) ion