Coordination Compounds

+3

To find the oxidation state of Co: (1) Let the oxidation state of Co = x. (2) NH₃ is a neutral ligand, so it contributes 0. (3) Each Cl⁻ contributes −1, and there are 2 Cl⁻ ligands, contributing −2. (4) The overall charge of the complex ion is +1. (5) Setting up the equation: x + 0 + (−2) = +1, therefore x = +3. So cobalt is in the +3 oxidation state, written as Co(III).

Linkage isomerism

Linkage isomerism arises because of ambidentate ligands — ligands that have two different donor atoms and can bind through either one. For example, NO₂⁻ can bind through nitrogen (giving −NO₂) or through oxygen (giving −ONO). Similarly, SCN⁻ can bind through S (M−SCN) or N (M−NCS). Geometric isomerism is about spatial arrangement (cis/trans). Coordination isomerism involves exchange of ligands between cationic and anionic metal centres. Ionisation isomerism involves exchange of ligands with counter ions.

Tetrahedral

In [Ni(CO)₄], nickel is in the zero oxidation state with electronic configuration 3d¹⁰. The hybridisation involves one s and three p orbitals (sp³), which gives a tetrahedral geometry. There are 4 CO ligands arranged at the corners of a tetrahedron. The complex is diamagnetic because all 3d electrons are paired in the d¹⁰ configuration. Square planar requires dsp² hybridisation (as in [Ni(CN)₄]²⁻), and octahedral requires 6 ligands. Trigonal bipyramidal involves 5 ligands like in Fe(CO)₅.

eg and t2g

In an octahedral crystal field: (1) The 5 degenerate d orbitals split into two groups. (2) The eg set contains dx²−y² and dz² orbitals — these point directly towards the ligands along the axes and experience MORE repulsion, so they are raised in energy by (3/5)Δo. (3) The t2g set contains dxy, dyz, and dxz orbitals — these point between the axes and experience LESS repulsion, so they are lowered in energy by (2/5)Δo. The energy gap between them is called crystal field splitting energy, Δo.