How to calculate the enthalpy of reaction using Gaussian 09?
In principle your approach is correct. There are a few pitfalls though. 

Quantum chemistry is usually not well equipped handling ions, especially anions, within reactions, hence the evaluation of these reaction energies will probably contain some errors. This is mostly due to the fact that you are technically calculating everything in the gas phase, and apply thermal corrections later. In well behaved systems, e.g. organic compounds without charges, that does not matter much. But even then we try to exploit error compensation with (for example) isodesmic reactions.

If you are treating the ions separately, you are neglecting all possible interactions between them, too. That might lead to additional errors. One way to approximate this is to calculate the ion pair together. That might not always lead to the correct result since solvents will also play a role here. Obviously you could also include these.

Another problem for your system is probably conformational space. Especially the phosphine is somewhat flexible and will adopt a few conformations (isolated as well as complexed). Here it is also important to find the global minimum, to evaluate the energies correctly.

It is advisable to look for similar systems in the literature and see what they have done. You may also find benchmark studies that will give you more information on how well your methodology performs for such systems. There is no general recipe for success; it always involves a lot of hands-on work. Eventually you will want to compare your calculated results to experimental ones to verify your findings.