Rovibrational FTIR spectra of Ni(CO)(4) and Fe(CO)(5) have been recorded in supersonic argon expansions at resolutions up to 0.004 cm(-1). Rotational temperatures as low as 4 K for the nu(6) band of Fe(CO)(5) and 6 K for the nu(5) band of Ni(CO)(4) have been determined from the populations of the rotational levels, and an efficient vibrational cooling has been evidenced by the large reduction of hot band absorptions, An analytical model including the instrumental line shapes and the intensity distribution is proposed to enable simulations of overlapping rovibrational transitions of symmetric top molecules. The spectroscopic study of the nu(6) and nu(10) stretching bands of Fe(CO)(5) yielded the -1 following parameters (3 sigma uncertainties in parenthesis) : nu(0)=2038.106 97(91) cm(-1), B-6=0,026 797(12) cm(-1), and B-0=0.026 826(10) cm(-1) for the 6(0)(1) transition, nu(0)=2015.5513(12) cm(-1), Delta A(10)=-0.000 086(5) cm(-1), B-10=0.026 786(4) cm(-1), and xi(10)=00.04(3) for the 10(0)(1) transition based on the literature value of A(0).