I was messing about earlier trying to figure out the most effective way to do this.
 
Essentially, if I was finishing off a music track and wanted to compress the whole master buss however wanted to leave the mid, treble frequencies alone so they are perceived in the mix. Eg. I want to leave anything after 2Khz alone and only compress before it.
So parallel compression but not every frequency.
 
I tried using an Auxiliary track with equaliser beforehand followed by a compressor which isn't sidechaining.
 
Also tried using a PC4K compressor however I wasn't that impressed with the results.
 
Now there must be an obvious way to do this however I'm not really sure if I'm missing something obvious. 
 
So to summarise  I am trying to setup a bus compressor which has a sidechain to an external eq/high cut (or eq setting) which only compresses material  2Khz and after.
 
 

why not use a multiband compressor on the bus and leave those bands alone which you don't want to compress?
 
if it has a mix control knob you could even blend it (parallel compression) without having to use a bus

That's an option I guess. More than one way to skin a cat.

I was messing about with a track and after some effort and tweaking I got some sort of results. In a way, the differences were subtle like more air in the mix without having increase or decrease reverb from the assorted elements. Also I changed my mastering chain a bit which meant not so much transient reduction and the mix was able to breath a bit more in the mid-treble range.
 

There are indeed many cat-skinning techniques. But each one yields different results.
 
Frequency-dependent compression can mean [at least] two different things: filtering the sidechain, or compressing specific frequency bands. Two very different effects, and I'm not sure which one you're after.
 
In the first method, the compressor acts normally (and can be either broadband or multi-band) but it's looking at a filtered sidechain when deciding how much to compress. That's normally done within the compressor, and requires that the plugin provide a sidechain filter. Many compressors do, although the sophistication of their filters varies. One plugin that's designed specifically for doing this on the master bus is the free TDR Kotelnikov bus compressor, although in the free version its sidechain filter is limited to high-pass. My favorite is Fabfilter's Pro-C, which offers a full parametric EQ for the sidechain filter.
 
In the second method, the main signal is split into two or more frequency bands, each of which gets its own compressor. The Sonitus Multiband works well for this, but you may find the newer LP-MB easier to use and potentially more transparent. 
 
There is a third option: dynamic equalization. My favorite tool for this is MDynamicEQ from Meldaproduction. Dynamic EQ avoids potential issues inherent in multiband compressors from splitting and recombining the signal, because it's an equalizer rather than a compressor. It's just as easy to use as a compressor, though.
 
Which of these methods one might choose depends on the desired effect. On the master bus, a common problem is when low frequencies dictate overall compression, which can, for example, cause a dominant kick to smack down an innocent tambourine. If that's your scenario, I'd recommend Cakewalk's LP-MB with a single band.

Thanks for the answer. Very informative and insightful.

OK so here is what I did and it's hardly eloquent either however it seemed to work.
 
I created a LP MB in the Master bus. Setup two bands, 1st band with no alterations, compression ratio etc. 2nd band with some light compression on the mids to highs. Set the sidechain to external.
 
Created a new stereo bus called submix. sent all the other relevant busses to submix. Muted the output, set the Send to LP-MB (Input 2). 
 
When going back into the LP-MB on the master bus using Sidechain Audition you can hear the audio being affected by the compression.
 
Not really elegant however the Master Output doesn't really offer much of external output to other busses that are not hardware. I had a look at the automation settings although the envelopes only address data in the same audio bus as far as I can tell anyway.
 

Somaltus with no preset then eq pray

i would think that a MBC on the master buss, with only partial freq's getting compressed,
would really screw a mix up.

Not sure if anything is really working as yet. Going to try something more basic. I have a mix I'm working on which should at least allow me to use eq and compression to make sonic room for a kick off. Might invest in fab pro c or anything similar.