An attempt to simplify (to make it a bit more comprehensible to a curious highschooler) and explain some of the terminologies used in <PERSON>'s answer. This is not intended to be a standalone answer and should not be considered as one; please read through the rest of the answers for a thorough explaination.
All coordination compounds have a net stabilizing energy called crystal field stabilization energy (CFSE), due to a crystal field effect with respect to a spherical symmetrical (think of ligands approaching the central metal atom (CMA) equally from all the directions, like a sphere) ligand field; CFSE always has a negative or a positive value and one of its fundamental uses is the prediction of stability of a complex (greater the magnitude of CFSE, greater is the stability). I use a simple formula to calculate CFSE:
$$ \pu{CFSE= -0.4\Delta_\circ (n_{t_2g}) + 0.6\Delta_\circ (n_{eg}) + n'P}$$
The formula doesn't hold much importance here, although the term $\Delta_\circ$ does. What's that? Glad you asked. Before we dive in, something about crystal field theory:
* The CMA is assumed to be at origin and the six ligands are approaching the CMA from $+x, -x, +y, -y, +z, -z$ directions. Now the ligands are approaching along the axis, and $d_{x^2-y^2}$ and $d_{z^2}$ orbitals have their lobes along the axis, so they:
* Face more repulsion from the incoming ligands, which increases their energy content. And did I tell you that they're codenamed $e_g$?
* Form a sigma bond, which explains:
When a $\ce{Cl-}$ ligand pi-donates into the metal center, it can only interact with the $\mathrm{d}{xy}$, $\mathrm{d}{xz}$, $\mathrm{d}{yz}$ $(t\mathrm{2g})$ orbitals because the $\mathrm{d}{x^2-y^2}$, $\mathrm{d}{z^2}$ $(e_{g}^*)$ orbitals have only a sigma-type interaction with the ligand $\dots$
* The remaining three orbitals (namely $d_{xy}, d_{yz}, d_{zx}$) are code-named $t_2g$ and are at a lower energy level.
This splitting up of the orbitals into two groups is called crystal field splitting. The energy difference between those two groups of orbitals is precisely what our $\Delta_\circ$ measures. Well, I've been talking about octahedral complexes all along, $\Delta_\circ$ has an evil brother $\Delta_t$ which goes for tetrahedral complexes. 976
So now we know: $\Delta_\circ$ is crystal field splitting energy. Did you say energy? I think I've studied this formula somewhere:
Energy = $\displaystyle \frac{hc}{\lambda}$
Seems good, let's apply it:
$\Delta_\circ = \displaystyle \frac{hc}{\lambda_{absorbed}}$
Wait! What's up with that 'absorbed' thing? Nothing much, but it appears that complexes seem to know some unicorn magic. So what they do is: they absorb (a wavelength corresponding to) a particular color and emit a wavelength that corresponds to another color. Don't worry, some noble souls have deciphered that, they follow this:
Absorb orange, emit indigo; absorb blue, emit red, etc... Green, however is a mysterious case. Also, please keep in mind that:
If $\Delta_\circ$ is less, $\lambda_{absorbed}$ is more and $\lambda_{observed}$ and vice-versa. Good to go. When <PERSON> says,
you reduce the octahedral field splitting energy $\Delta_\circ$ which redshifts the absorbance of the compound.
They mean that a higher wavelength of light (on a blue-colored tone) must've been absorbed (reddish colors have a higher wavelength).
Why does $\Delta_\circ$ decrease?
Excerpts and images that follow are directly from <PERSON> et al. Content is copyrighted to their respective owners.
Understanding this can be quite tricky unless you're acquainted with angular overlap method. Nevertheless, let's give it a shot.
What's angular overlap method?
This is a method of estimating the relative magnitudes of molecular orbital energies within coordination complexes. 70