In situ infrared spectroelectrochemistry studies of ferrocene, [Rh2(dimen)2(dppm)2](PF6)2 and (mes)Cr(CO)3 (dimen = 1,8-diisocyanomenthane; dppm = bis(diphenylphosphino)methane); mes = mesitylene). A useful technique for the characterization of electrochemically generated organometallic species