Treatment of [CpCr(NO)I](2) with an excess of a Lewis base, L, in CH2Cl2 leads to the formation of the complex salts [CpCr(NO)(L)(2)](+)[I](-) ([1]I-+(-), L = NH3; [3]I-+(-), L = NH2CH2CH=CH2; [7]I-+(-), L = 1/2en). Heating of salts [1]I-+(-) and [3]I-+(-) results in loss of L and formation of the neutral complexes, CpCr(NO)(NH(2)R)I (2, R = H; 4, R = CH2CH=CH2), respectively. In contrast, reaction of [CpCr(NO)I](2) with the bulkier NH(2)CMe(3) affords the neutral CpCr(NO)(NH(2)CMe(3))I (6) directly. Sequential reaction of 6 or CpCr(NO)(P{OMe}(3))I with AgPF6 and further L affords respectively the salts [CpCr(NO)(L)(2)](+)[PF6](-) ([5](+)[PF6](-), L = NH(2)CMe(3); [8](+)[PF6](-), L = P(OMe)(3)). All these species exhibit room-temperature ESR spectra and magnetic moments consistent with their possessing 17-valence-electron configurations. Zinc reduction of [CpCr(NO)I](2) in the presence of P(OMe)3 leads to the improved synthesis of the known complex CpCr(NO)(P{OMe}(3))(2) (8), and a similar reduction with CNCMe(3) affords the previously unknown CpCr(NO)(CNCMe(3))(2) (9). The solid-state molecular structures of [1](+)[BPh(4)](-). NCMe, 4, 8, and [8](+)[BPh(4)](-) have been -crystal X-ray crystallographic analyses which afforded the following data. [CpCr(NO)(NH3)(2)][BPh(4)].NCMe ([1](+)[BPh(4)](-). NCMe): monoclinic, space group P2(1)/n; Z = 4; a 9.478(3) Angstrom; b = 19.288(7) Angstrom; c = 15.427(6) Angstrom; beta = 91.99(3)degrees; V = 2818.5 Angstrom(3); T = 200 K; R(F) = 0.038 for 2185 data (I-o greater than or equal to 2.5o(I-o)) and 310 variables. CpCr(NO)(NH2C3H5)(I) (4): triclinic, space group P (1) over bar; Z = 2; a = 8.0497(8) Angstrom; b = 8.3273(17) Angstrom; c = 9.3284(9) Angstrom; alpha = 108.182(12)degrees; beta = 92.370(8)degrees; gamma = 94.759(12)degrees; V = 590.54 Angstrom(3); T = 295 K; R(F) = 0.024 for 1756 data (I-o greater than or equal to 2.5 sigma(I-o)) and 123 variables. CpCr(NO)(P{OMe}(3))(2) (8): monoclinic, space group P2(1)/a; Z = 8;(1)a = 18.080(4) Angstrom; b = 9.320(4) Angstrom; c = 21.068(3) Angstrom; beta = 93.02(2)degrees; V = 3545.1 Angstrom(3); T = 205 K; R(F) 0.040 for 3356 data (I-o greater than or equal to 2.5 sigma(I-o)) and 411 variables.[CpCr(NO)(P{OMe}(3))(2)][BP4] ([8](+)[BPh(4)](-)): monoclinic, space group P2(1)/n; Z = 4; a 10.086(2) Angstrom; b = 22.253(3) Angstrom; c 16.150(4) Angstrom; beta 90.42(2)degrees; V = 3624.7 Angstrom(3); T = 195 K; R(F) = 0.044 for 3334 data (I-o greater than or equal to 2.5 sigma(I-o)) and 449 variables. Despite its 17-electron configuration, [1](+) does not undergo ligand substitution, nor does it effect H-atom abstraction from HSn(n-Bu)(3). However, it exhibits an irreversible 3 V vs SCE in THF, and zinc reduction of [1](+) (as its [PF6](-) salt) in the presence of CO (1 atm) affords CpCr(NO)(CO)(2). In a reverse manner, oxidation of 2 by [Cp(2)Fe](+)[PF6](-) in acetonitrile produces [CpCr(NO)(NCMe)(2)](+)[PF6](-) a salt which contains a 17-electron cation similar to [1](+). These experimental observations lead to the conclusion that for CpCr(NO)L(2) complexes, sigma-base ligands stabilize the 17-electron configurations of cations whereas pi-acid ligands stabilize the 18-electron configurations of the neutral congeners. Intermediate ligands (e.g. L = P(OMe)(3)) yield complexes which are capable of existing in both forms. This trend can be rationalized by the results of an Extended Huckel analysis of the CpCr(NO) fragment and the interaction of its frontier orbitals with those of various ligands, L.