The homoleptic Co(-I) isonitrile complex [Co(CNPh*)(4)](-) (1(-), Ph* = 2,6-C(6)H(3)Me(2)), the first example of an isonitrilate complex, can be prepared by ethylene substitution within the Co(-I) complex [CO(C2H4)(4)](-) or by potassium naphthalenide (K(Nap)) reduction of Co(O), Co(I), and Co(II) substrates. Reductions of the isonitrile-containing precursors [Co(CNPh*)(4)I-2] (3) and [Co(CNPh*)(5)](+) (4(+)) proceed via [Co-2(CNPh*)(8)] (2), which is then reduced further to 1(-). Complex 1(-) can be prepared in one pot by K(Nap) reduction of cobaltocene followed by addition of CNPh* or by potassium reduction of a mixture of CoI2 and CNPh* to give 2, which is then further reduced to 1(-). The anion 1(-) can be isolated as K+, [K(DME)](+), [K(crypt-2,2,2)](+), and [K(18-C-6)](+) salts and has been characterized spectroscopically and analytically. Oxidation of 1(-) gives 2, which is the only observable product from the reaction of 1(-) with many simple electrophiles. The only electrophilic adduct of 1(-) isolated to date is [Co(CNPh*)(4)(SnPh(3))] (5), the product of reaction with Ph(3)SnCl. Single-crystal X-ray diffraction has confirmed that [K(DME)]1 contains discrete, approximately tetrahedral cobalt anions held together as loose dimers through interactions with the [K(DME)](+) counterions (space group , with a = 11.160(2) Angstrom, b = 14.204(3) Angstrom, c = 14.339(3) Angstrom, alpha = 102.91(1)degrees, beta = 11.88(1)degrees, gamma = 102.78(1)degrees, D-c = 1.223 g/mL, Z = 2, and R(F) = 5.93%). Crystallography has also established that 2 is a dimer with two bridging isonitrile groups (space group , with a = 12.239(4) Angstrom, b = 13.985(4) Angstrom, c = 21.609(6) Angstrom, alpha = 98.87(2)degrees, beta = 99.19(2)degrees, gamma = 112.72(2)degrees, D-c = 1.185 g/mL, Z = 2, and R(F) = 6.10%), and examination of molecular parameters for 1(-) and 2 suggest that increased back-donation of electron density from the metal results in significant lengthening of the C=N triple bond in 1(-) but has no consistent effect on the CNC bend angles of the ligands. Decreases in the CNC angles correlate instead with close approaches between the carbon atoms of the ligands and the [K(DME)](+) counterions, which are also responsible for the major distortions of the metal geometry away from that of an ideal tetrahedron.