A new series of Fe(II) complexes, FeCl2{N(R)=C(Me)C(Me)=N(R)}, containing diimine ligands with hemilabile sidearms R (R = CH2(CH2)(2)NMe2, 1, CH2(CH2)(2)OMe, 2, CH2(CH2)(2)SMe), 3) were synthesized. The crystal structure of 1 showed 6-coordination where both amine arms were attached, whereas 2 was a 5-coordinate 16e species with one methoxy arm dangling free. Extensive attempts were made to bind CO to these species to synthesize precursors for dihydrogen complexes but were unsuccessful. Reaction of I with 1 or 2 equiv of AgOTf under CO atmosphere resulted in isolation of only a 6-coordinate bis(triflate)-containing product [Fe{N(R)=C(Me)C(Me)= N(R)}(OTf)(2)] (R = CH2(CH2)(2)NMe2), 5. Reaction of 5-coordinate 2 with AgSbF6 under CO did not give a CO adduct but afforded instead a dicationic dinuclear complex [Fe{N(R)=C(Me)C(Me)=N(R)}(mu-Cl)](2)[SbF6](2) (R = CH2(CH2)(2)OMe), 4, containing a weakly bound SbF6. Thus coordination of hard-donor anions to iron was favored over CO binding. The unexpected rejection of binding of CO is rationalized by the iron being in a high-spin state in this system and energetically incapable of spin crossover to a low-spin state. Theoretical calculations on CO interaction with Fe(II) centers in spin states S = 0, 1, and 2 for both the 16e complexes and their CO adducts aid further understanding of this problem. They show that interaction of CO with a high-spin 5-coordinate Fe model diimine complex is essentially thermoneutral but is exergonic by about 48 kcal/mol to a comparable but low-spin diphosphine fragment. Spin crossover is thus disfavored thermodynamically rather than kinetically (e.g. a "spin block" effect); i.e., the ligand field strengths of the primarily N-donor groups are apparently insufficient to give a low-spin CO adduct.