Complexes with Zintl ions as ligands possess intriguing structures. An early study repor...

Complexes with Zintl ions as ligands possess intriguing structures. An early study reported that reaction of

K₃E₇ (E = P, As, Sb) with sources of M(CO)₃ (M = Cr, Mo, W), in the presence of cryptand [2.2.2] results in [K(2.2.2)]₃[M(CO)₃(E₇)] (S. Charles, B. W. Eichhorn, A. L. Rheingold, S. G. Bott, J. Am. Chem. Soc., 1994, 116, 8077). Why do these reactions require cryptand[2.2.2] to be successful? Discuss the relative p-donor abilities of these Zintl ions to the Cr(CO)₃ fragment (Hint: use IR data). What trend is observed in the electronic spectra for the [K(2.2.2)]₃[Cr(CO)₃(E₇)] series? Briefly explain why this trend seems reasonable on the basis of how it is assigned. These trianions can be protonated to afford dianions. Sketch [Cr(CO)₃(HSb₇)]^2- to clearly indicate the protonated site.