SSRS IF Statement with incorrect syntax

I'm new to SSRS and find the syntax very difficult as well as the awful expression text box that makes it very hard to see where i've made errors.

I want to do IFF (First(Fields!TaxNumber.Value, "InvoiceHeader") = null, then only print IS02Chars.value, else print both IS02Chars and TaxNumber.Value.

  =IFF (First(Fields!TaxNumber.Value, "InvoiceHeader") = null , ": " & First(Fields!ISO2Chars.Value, "InvoiceHeader"), ": " &
  First(Fields!ISO2Chars.Value, "InvoiceHeader") &
  First(Fields!TaxNumber.Value, "InvoiceHeader"))