Reduction of (PNP)MCI [PNP = ((Bu2PCH2SiMe2)-Bu-t)(2)N] with Mg gives three-coordinate, T-shaped (PNP)M for M = Fe(S = 3/2) and Ni. Their reactivity was tested toward CO; Ni binds one CO, but only reversibly (i.e., CO is completely lost in vacuum), and has a CO stretching frequency showing effective back-donation by Nil. The structure of (PNP)Ni(CO) is intermediate between planar and tetrahedral, in contrast to the planar d(8) analogue, (PNP)Co(CO), This structural reorganization on carbonylation changes the singly occupied molecular orbital from having negligible phosphorus character [no P hyperfine structure in the electron paramagnetic resonance (EPR) spectrum of (PNP)Ni] to having enough P character to have a triplet structure in the EPR spectrum of the CO. The presence of one fewer electron in (PNP)Fe (vs the Co analogue) leads to binding of two CO, and (PNP)Fe(CO)2 is characterized as a spin doublet with square-pyramidal structure. Density functional theory calculations strengthen the understanding of the structural and spectroscopic changes along this d(n) series (n = 7-9).