Structural and spectroscopic comparisons of the square-planar four-coordinate [o-phenylenebis(biuretato)]cobaltate(III) complex and the five-coordinate mono- and six-coordinate dicyano adducts

The addition of cyanide ion to the paramagnetic four-coordinate square-planar cobalt(III) complex, [Co(bbphen)]^- {H₄bbphen = o-phenylenebis(biuret), [1]^-}, afforded the diamagnetic mono- and dicyano complexes [Co(bbphen)(CN)]^2- ([2]^2-) and [Co(bbphen)(CN)₂]^3- ([3]^3-). The X-ray crystal structure analyses of [K(crypt)][1]) and (crypt = cryptand[2,2,2]), (PPh₄)₂[2]·2CH₃CN·H ₂O, and K₃[3]·5.5H₂O show them to be a four-coordinate square plane, a five-coordinate square pyramid and a six-coordinate octahedron, respectively. The four-coordinate N atoms of the bbphen ligand in the three complexes were located in a basal plane around the central cobalt atom. The average Co-N bond lengths become longer with increasing coordination number. A novel interactive contact between one of the potassium ions and both the cyano ligand and the phenyl ring of the bbphen ligand is found in K₃[3]·5.5H₂O. Complex [2]^2- exhibits characteristic intense CT absorption bands at 14.0 × 10³ (ε = 1380) and 22.7 × 10³ cm^-1 (ε = 1480 dm³ mol^-1 cm^-1) together with the distinctively high field ¹³C NMR chemical shift of the coordinated cyanide ion at 81.5 ppm, as compared with 143.0 ppm for [3]^3- in DMSO.