An assessment of the cis–influence in coenzime B12 models. The structure of the mixed Schiff–base/oxime complex: aquo(3,9–dimethyl–2,10–diethyl–1,4,8,11–tetra–aza–undeca–1,3,8,10–tetraen–11–ol–1–olato) (methyl)cobalt(III) hexafluorophosphate