We study the electronic structures and magnetic properties of

{Mn}_{2} Co Z

(Z = Al, Ga, In, Si, Ge, Sn, Sb)

compounds with

{Hg}_{2} Cu Ti

-type structure using first-principles full-potential linearized-augmented plane-wave calculations. It is found that the compounds with

Z = Al

, Si, Ge, Sn, and Sb are half-metallic ferrimagnet. Experimentally, we successfully synthesized the

{Mn}_{2} Co Z

(Z = Al, Ga, In, Ge, Sn, Sb)

compounds. Using the x-ray diffraction method and Rietveld refinement, we confirm that these compounds form

{Hg}_{2} Cu Ti

-type structure instead of the conventional

L 2_{1}

structure. Based on the analysis on the electronic structures, we find that there are two mechanisms to induce the minority-spin band gap near the Fermi level, but only the

d − d

band gap determines the final width of the band gap. The magnetic interaction is quite complex in these alloys. It is the hybridization between the

Mn (C)

and Co atom that dominates the magnitude of magnetic moment of the Co atom and the sign of the

Mn (B) − Co

exchange interaction. The

{Mn}_{2} Co Z

alloys follow the Slater-Pauling rule

M_{H} = N_{V} - 24

with varying

Z

atom. It was further elucidated that the molecular magnetic moment

M_{H}

increases with increasing valence concentration only by decreasing the antiparallel magnetic moment of

Mn (C)

, while the magnetic moments of

Mn (B)

and Co are unaffected.

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