We present a detailed experimental and theoretical study concerning bonding and structural stability of intermetallic electron compounds with the

{PtHg}_{4}

structure. Due to the simplicity of the structure these compounds represent an excellent prototype system for a more general insight into bonding and stability of the large family of

d - s p

bonded Hume-Rothery compounds. In particular, the representatives

{CrGa}_{4}

and

{MnGa}_{4}

were synthesised and their resistivity, magnetic susceptibility, and bulk modulus measured. We find that both compounds are metallic conductors but show a remarkable large difference in their temperature independent magnetic susceptibilities. The value of the Pauli paramagnetic susceptibility of

{MnGa}_{4}

is about

5 \times 10^{- 9}

m^{3} / mol

higher than that of

{CrGa}_{4} .

The

{PtHg}_{4}

structure of

{CrGa}_{4}

and

{MnGa}_{4}

is stable up to pressures of about 100 kbar. Full-potential linearized augmented plane wave calculations reproduced very well the experimental structural properties of

{CrGa}_{4}

and

{MnGa}_{4}

and showed strong directional (covalent) bonding between transition metal atoms and Ga atoms in both compounds. The directional bonding is due to a large hybridization of the narrow d bands with the Ga

sp

bands. As a consequence a large pseudogap at the Fermi level for

{CrGa}_{4}

and slightly above the Fermi level for

{MnGa}_{4}

is produced. This pseudogap is characteristic and decisive for structural stability of electron compounds with the

{PtHg}_{4}

structure. We find that structural stability appears as a competition between optimizing the pseudogap and minimizing the compound equilibrium volume. Therefore, stable electron compounds are confined to systems

T {Ga}_{4}

with T being a transition metal from group 6 or 7. A complete substitution of Ga for isovalent Al or In is not possible.

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